Abstract:
A fuel cell stack includes at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current; a sensor cell having an anode, a cathode and a membrane between the anode and the cathode, the anode being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected across the stack to carry the electric current whereby hydrogen from the portion of fuel is electrochemically pumped to the cathode of the sensor cell; and a sensor communicated with the sensor cell to receive a signal corresponding to evolution of hydrogen from the anode to the cathode of the sensor cell and adapted to detect contaminants in the fuel based upon the signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to fuel cells and, more particularly, to improving detection of contaminants entering the fuel cell through the reactant streams and their subsequent removal from the system. 
         [0002]    Poor reactant and water quality can adversely affect the performance of fuel cells. Fuel can frequently carry contaminants which may poison the catalyst of the fuel cell. Such contaminants include carbon monoxide, sulfur dioxide, hydrogen sulfide and the like. Foreign cations in the water, which is used to cool and humidify the fuel cell, can deposit in the cell and interfere with its proper operation. Foreign cations may also be introduced to the cell via the reactant streams, e.g. sodium chloride in air 
         [0003]    Fortunately, fuel cells and especially PEM fuel cells have a demonstrated ability to recover from contamination. For example, catalyst poisons as discussed above can be removed, and catalyst activity recovered, by raising the anode potential to close to the air potential. This is accomplished in various methods including that which is disclosed in U.S. Pat. No. 6,841,278. 
         [0004]    Foreign cations in the water can be removed through the water transfer plates, for example, by flushing the contaminated cells with clean water. 
         [0005]    Although proper measures can be taken, nothing known in the art helps identify when such measures should be taken. Thus, such maintenance can to date only be done on a regularly scheduled basis, which clearly runs the risk of conducting such steps either when not needed or after performance of the fuel cell stack is already deteriorating, neither of which is desirable. 
         [0006]    It is clear that the need exists for good indication as to when corrective or cleaning procedures based upon contaminants are needed. 
         [0007]    It is therefore the primary object of the present invention to provide a system and method which identify when contaminants are present. 
         [0008]    It is a further object of the invention to provide such identification in a system which is simple and reliable in practice, and which does not add significantly to the components or cost and/or weight of such components, of the fuel cell stack or power plant into which such measures are implemented. 
         [0009]    Other objects and advantages of the present invention will appear below. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with the invention, the foregoing objects and advantages have been readily attained. 
         [0011]    According to the invention, a fuel cell stack is provided which comprises at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current; a sensor cell having an anode, a cathode and a membrane between the anode and the cathode, the anode being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected across the stack to carry the electric current whereby hydrogen from the portion of fuel is evolved to the cathode of the sensor cell; and a sensor communicated with the sensor cell to receive a signal corresponding to electrochemical pumping of hydrogen from the anode to the cathode of the sensor cell and adapted to detect contaminants in the fuel based upon the signal. 
         [0012]    In further accordance with the invention, a method is provided for operating a fuel cell power plant, which method comprises operating a fuel cell stack comprising at least one fuel cell having a fuel inlet for directing a hydrogen fuel to the fuel cell to generate electric current, and a sensor cell having an anode, a cathode and a membrane between the anode and the cathode, the anode being communicated with the fuel inlet to receive a portion of fuel from the fuel inlet, the sensor cell being connected across the stack to carry the electric current whereby hydrogen from the portion of fuel is electrochemically pumped from the anode to the cathode of the sensor cell; and monitoring a parameter corresponding to electrochemical pumping of hydrogen from the anode to the cathode of the sensor cell to detect contaminants in the reactants based upon variation of the parameter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A detailed description of preferred embodiments of the present invention follows, with reference to the attached drawings, wherein: 
           [0014]      FIG. 1  schematically illustrates a system according to the invention; 
           [0015]      FIG. 2  illustrates an alternative embodiment utilizing two different sensor cells; 
           [0016]      FIG. 3  illustrates potential difference vs. current density for normal, low hydrogen and high CO operation; and 
           [0017]      FIG. 4  illustrates a two-pass fuel flow system utilizing the contaminant detection according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The invention relates to detection of contaminants present in a fuel cell, especially to a PEM fuel cell. 
         [0019]      FIG. 1  shows a fuel cell stack  10  which includes a plurality of fuel cells  12  (only one illustrated in  FIG. 1 ) and a sensor cell  14 . Fuel cells  12  have an anode flow field or anode  16  and a cathode flow field or cathode  18 , and a membrane  20  disposed between them in well known manner. Hydrogen or a hydrogen-containing fuel is fed through a fuel inlet  22  to anode  16  of fuel cells  12  while oxidant is fed through an oxidant inlet  24  to cathode  18  of fuel cells  12 , also in well known manner, to generate electricity across stack  10 . Exhaust exits anode  16  through anode exhaust  26 , while exhaust exits cathode  18  through cathode exhaust  28 . 
         [0020]    Sensor cell  14  also has an anode  30  and a cathode  32  as well as a membrane  34  positioned between anode  30  and cathode  32 . Unlike fuel cells  12 , sensor cell  14  is operated by feeding fuel only, and stack current passing through sensor cell  14  drives hydrogen across membrane  34  so that pure hydrogen is evolved at cathode  32 . This operation of sensor cell  14  is sometimes referred to as operating as an electrochemical hydrogen pump. 
         [0021]    Operation of sensor cell  14  as a hydrogen pump makes the cell far more sensitive to contaminants in the fuel cells, for example contaminants introduced via the reactant streams, and thus this cell can advantageously be used to detect such conditions, and to correct for them, at the proper time and without either unneeded shutdowns and/or impaired performance of the stack as happens with only regularly scheduled contaminant purging procedures. 
         [0022]    According to the invention, sensor cell  14  receives fuel at anode  30  through a sensor fuel inlet  36  which is communicated with fuel inlet  22  of fuel cells  12 . No oxidant is fed to cathode  32 , and current and potential from the stack serves to drive hydrogen across membrane  34  as described above. Exhaust from anode  30 , if any, can be fed to anode exhaust  26  of fuel cell  12 . Exhaust from cathode  32  is substantially pure hydrogen, and can advantageously be further used in accordance with the invention. Thus, this exhaust is not merely vented, but can be recycled back to a fuel inlet as shown in  FIG. 1 . Alternatively, exhaust from cathode  32  can be fed through an exhaust line  33  back to fuel inlet  22  at a point downstream from the branch from fuel inlet  22  which feeds sensor cell  14 . In this way, fuel used in sensor cell  14  is not wasted and is used in a manner which does not adversely affect the function of sensor cell  14 . If this hydrogen were fed to the inlet feeding sensor cell  14 , the pure hydrogen may mask potential contaminants and increase the hydrogen concentration in the fuel and reduce the ability of sensor cell  14  to detect same. 
         [0023]    Still referring to  FIG. 1 , a water loop  38  can be provided for feeding water as coolant and humidifier to fuel cells  12  and, if desired, to sensor cell  14 . This water can be fed to a water transport plate  40  as shown, which advantageously conveys water as needed to fuel cells  12 . As shown, a water transport plate  42  can also be provided along sensor cell  14 . This is less likely to be needed for cooling, but is useful for managing water content in the sensor cell  14 . It should be noted that a water transport plate would also preferably be present between cells, and for example between cathode  18  and anode  30 . Such additional water transport plates are not shown in the drawings for the purpose of simplicity. 
         [0024]    In accordance with the invention, sensor cell  14  is advantageously more sensitive than the rest of stack  10  to deactivation due to catalyst poisons such as CO and Sulfur compounds which can be present in the fuel, and also to cation contamination from coolant water and the like. Thus, by monitoring a parameter related to rate of electrochemical pumping of hydrogen across membrane  34 , sudden changes in such parameter can be indicative of the contaminants which are to be detected according to the invention. 
         [0025]    For example, if fuel in fuel inlet  22  contains carbon monoxide, while this carbon monoxide will affect all cells  12 ,  14 , it will affect cell  14  more significantly due to operation of cell  14  in a hydrogen pump mode. Deactivation of the catalyst in cell  14  will result in an increase in the potential difference across sensor cell  14 , which will be a larger percentage of the sensor cell  14  potential than the potential of fuel cell  12  and this significant increase in potential difference is one parameter which can advantageously be monitored for change according to the invention. This catalyst activity loss will be relatively independent of the current density of the cells  12 , 14 . The presence of cations in the membrane of the fuel cells  12 ,  14  will also cause an increase in potential difference across sensor cell  14  due to increased ohmic losses. However, since this change is related to changes which are proportional to current density, the class of contaminant can be determined by deliberately altering the stack current and watching the potential difference across sensor cell  14 . 
         [0026]    In order to measure the desired parameter, a sensor  44  can be provided for determining potential difference between the anode and cathode sides of sensor cell  14 .  FIG. 1  shows this as a simple voltage meter, but it should be appreciated that a wide variety of parameters can be measured, and a further wide variety of instruments can be used to make such measurements, well within the scope of the present invention. 
         [0027]    In accordance with the invention, it is desired for sensor cell  14  to be as sensitive as possible so that contaminants can be detected well prior to any impact the contaminants can have on performance of the stack. In this regard, since the potential of the hydrogen sensor cell consists of just the polarization of the hydrogen reaction (both reduction and oxidation) and the resistance of the cell, and since these are very small relative to the cathode reaction (oxygen reduction reaction), the sensor cell is very sensitive to any change in either the hydrogen polarization or cell resistance. If a catalyst poison is present in the fuel, the potential of the sensor cell will increase due to increased activation polarization of the hydrogen reactions on the electrodes. 
         [0028]    On the other hand, if foreign cations are accumulating in the system, for example from accumulation within the system faster than they are removed, the resistance of the cells will increase in an amount proportional to the current density. Since such losses will be a larger percentage of the voltage of the sensor cell than the voltage of the fuel cells, the sensor cell will be the most sensitive to these losses as desired. 
         [0029]    Further, as set forth above, the type of contamination can be differentiated by how the increase in potential varies with current density. 
         [0030]    Suitable steps to take in response to contamination can include checking of fuel, air and/or water quality, initiation of shutdowns, activation of recovery procedures such as raising of the potential of the anode to remove the catalyst poison, or changing of DI beds or fuel filters, and the like. Further, in systems having air bleed capability, an increase in air bleed can also be a suitable step. 
         [0031]    It should also be noted that the signal measured from sensor cell  14  can also be used to determine whether the implemented remedial steps are having the desired effect. 
         [0032]    In accordance with one particular embodiment of the present invention, desirable results are obtained using two sensor cells.  FIG. 2  shows a stack  10  according to the invention, with like numerals showing similar elements as  FIG. 1 . In this embodiment, however, two sensor cells  14 ,  46  are included, and these cells can provide even further information regarding contaminants. 
         [0033]    According to the invention, sensor cell  14  and sensor cell  46  are provided having different catalysts, and these catalysts are selected preferably so that they react differently to contaminants. For example, platinum catalyst is particularly vulnerable to CO poisoning, while platinum/ruthenium catalyst is much less affected. According to the embodiment of  FIG. 2 , sensor cell  14  is provided using platinum catalyst on both electrodes, while sensor cell  46  is provided having PtRu catalyst on the anode electrode. Under normal circumstances, both sensor cells  14 ,  46  should read approximately the same difference in voltage. 
         [0034]    However, if fuel contamination is present, for example as CO, the CO poisons the Pt catalyst much more severely than the PtRu catalyst, and the Pt anodes will polarize much more than the cell which has Ru on the anodes. Thus, monitoring the voltage difference between the two cells can provide excellent information as to contaminants in the fuel cells. 
         [0035]    Referring to  FIG. 3 , measurements were made with a system having two different sensor cells as described above. The results shown in  FIG. 3  are the potential difference between the two sensor cells as a function of current density for normal and high CO operation, as well as a low hydrogen situation. 
         [0036]    In accordance with this embodiment of the invention, the anode electrode of sensor cell  46  is provided with CO tolerant catalyst, typically, PtRu catalyst. Other catalysts and catalyst combinations which would be useful with the present invention are disclosed in U.S. Pat. No. 5,183,713, incorporated herein in its entirety by reference. 
         [0037]    As with the embodiment of  FIG. 1 , when sensors  14 ,  46  indicate that contaminant is present, appropriate remedial steps can advantageously be taken so as to avoid impact upon efficiency of operation of the stack, and further to avoid the need for scheduled maintenance which could result in unneeded steps being taken. 
         [0038]    As set forth above, it is desired to make sensor cell  14  (and sensor cell  46  if included) as sensitive as possible so that warning can be given before any impact on efficiency takes place. In this regard, in a multiple pass fuel flow field design, the sensor cell can be integrated into the flow so that the sensor cell has one less pass than the other cells.  FIG. 4  shows such a system, and schematically illustrates a stack  10  having a plurality of fuel cells  12  (not individually shown) and a sensor cell  14 . Fuel is fed through a manifold from fuel inlet  22  to fuel cells  12  and sensor cell  14 . As schematically shown, oxidant such as air is also fed to fuel cells  12 . Cathode exhaust exits stack  10  through exhaust line  48 , while anode exhaust passes to a fuel turn manifold (schematically illustrated at  50 ) for a further pass through fuel cells  12 . According to the invention, the anode exhaust from sensor cell  14  is fed to turn manifold  50  so that the electrochemically pumped hydrogen is not wasted and is nevertheless not injected into the system in a location which interferes with proper operation of sensor  14 . Anode exhaust from the second pass through fuel cells  12  exits stack  10  at exhaust line  52 . This same type of setup can be implemented with stack configurations having 3 or more passes as well, and will result in the sensor cell always having one less pass than the other cells of the stack. This advantageously results in the sensor cell operating at a higher per pass fuel utilization than any other cell, which renders the cell most sensitive to detecting the contaminants discussed. 
         [0039]    It should be appreciated that the system and method of the present invention advantageously provide for detection of fuel cell operation which requires attention well prior to adverse impact upon operation of the system overall. 
         [0040]    While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.