Abstract:
A method for periodically removing water from cathode flow channels in a fuel cell stack that includes looking at the resulting cell voltage patterns in response to selectively pulsing the cathode airflow during. If the fuel cell stack has been in an extended low power condition for a predetermined period of time, the cathode airflow is pulsed, and the output voltage of each cell is measured to determine the difference between the cell voltages. If the cell voltages significantly vary, then the cathode airflow is pulsed more frequently, and if the cell voltages cells are sufficiently close, then the cathode air is pulsed less frequently. The propose water management diagnosis can be used in a control system to determine the frequency of cathode air pulsing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to a method for providing water management control in a fuel cell stack and, more particularly, to a method for determining how often to pulse a cathode airflow to remove water from cathode flow channels in a fuel cell stack during a sustained low power condition to provide low cell-to-cell output voltage variability.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines.  
         [0005]     A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.  
         [0006]     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).  
         [0007]     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.  
         [0008]     The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode reactant gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode reactant gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.  
         [0009]     As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. During continuous low stack power demands, typically below 0.2 A/cm 2 , water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, the flow channel may close off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution, creating an unstable stack operation and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked with water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.  
         [0010]     It is usually possible to purge the accumulated water in the flow channels by periodically forcing or pulsing the reactant gas through the flow channels at a higher flow rate than is necessary to provide the desired output power. For example, it is known in the art to pulse the cathode air through the flow channels when the fuel cell stack has been continuously operating at low power demands, such as would occur when the vehicle is idling for an extended period of time. For example, the cathode air may be pulsed to a level for half stack power every 3-5 minutes. By pulsing the cathode air in this manner, water is forced out of the channels. However, the increased airflow dries the membranes causing problems with expansion and shrinkage of the membrane. Also, an increased airflow increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency.  
         [0011]     Providing water management at the stack level does not necessarily translate to water management in all of the fuel cells in the stack. In other words, the water accumulation in the cathode flow channels may affect the cell output voltages differently, which is not addressed by known fuel cell water management processes. Cell variability may be the result of tolerances, aging of the stack or other factors. The variability in the state of hydration from cell-to-cell results in various problems, such as low power instability and low performing cells. Small variability in design and assembly of cells results in different pressure drops in the cathode flow field, anode flow field and the coolant flow field. This in turn causes variability in cell stoichiometry and temperatures. The relative humidity of gases in the flow field is very sensitive to stoichiometry and temperature.  
         [0012]     If the normal operating range of a fuel cell stack is 80-90% relative humidity, the cell-to-cell variability could result in some cells having a relative humidity over 100%, consequently flooding the cell. Moreover, when a cell partially floods it causes more pressure drop and a reduction of stoichiometry, thus resulting in a runaway condition leading to stack failure, possibly resulting in stack shutdown. The dynamics of this runaway is a function of cell-to-cell variability.  
         [0013]     Further, as the number of cells in the stack increases, these problems compound. No matter how tight the tolerances in the cells are, there still will be variability caused by plate design, MEA variability, diffusion media variability and assembly. Thus, there is a need for operational approaches to diagnose the extent of variability in the cells and develop a remedial action to minimize or reset the variability.  
       SUMMARY OF THE INVENTION  
       [0014]     In accordance with the teachings of the present invention, a method for periodically removing water from cathode flow channels in a fuel cell stack is disclosed that includes selectively pulsing the cathode airflow during extended low power load conditions, where the frequency of the pulsing depends on the cell-to-cell output voltage variability of the fuel cells in the stack during the pulse. If the fuel cell stack has been in the extended low power condition for a predetermined period of time, the cathode airflow is pulsed, and the output voltage of each cell is measured during the pulse to determine the difference between the cell voltages. If the cell voltages significantly vary, then the cathode airflow is pulsed more frequently, and if the cell voltages cells are nearly the same, then the cathode air is pulsed less frequently.  
         [0015]     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a cross-sectional view of a fuel cell in a fuel cell stack;  
         [0017]      FIG. 2  is a flow chart diagram showing a process for pulsing the cathode airflow during a low power stack output, according to an embodiment of the present invention;  
         [0018]      FIG. 3  is a graph with cell voltage on the vertical axis and time on the horizontal axis showing the output voltage potential of each cell in a fuel cell stack where the cell-to-cell variability is large; and  
         [0019]      FIG. 4  is a graph with cell voltage on the vertical axis and time on the horizontal axis showing the output voltage potential of each cell in a fuel cell stack where the cell-to-cell variability is small. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0020]     The following discussion of the embodiments of the invention directed to a method for determining cell output voltage variability in a fuel cell stack to determine how often the cathode airflow will be pulsed during extended low power operating conditions to remove water from the cathode flow channels is merely exemplary in nature, and is in no way intended the limit the invention or its applications or uses.  
         [0021]      FIG. 1  is a cross-sectional view of a fuel cell  10  that is part of a fuel cell stack of the type discussed above. The fuel cell  10  includes a cathode side  12  and an anode side  14  separated by an electrolyte membrane  16 . A cathode side diffusion media layer  20  is provided on the cathode side  12 , and a cathode side catalyst layer  22  is provided between the membrane  16  and the diffusion media layer  20 . Likewise, an anode side diffusion media layer  24  is provided on the anode side  14 , and an anode side catalyst layer  26  is provided between the membrane  16  and the diffusion media layer  24 . The catalyst layers  22  and  26  and the membrane  16  define an MEA. The diffusion media layers  20  and  24  are porous layers that provide for input gas transport to and water transport from the MEA. Various techniques are known in the art for depositing the catalyst layers  22  and  26  on the diffusion media layers  20  and  24 , respectively, or on the membrane  16 .  
         [0022]     A cathode side flow field plate or bipolar plate  18  is provided on the cathode side  12  and an anode side flow field plate or bipolar plate  30  is provided on the anode side  14 . The bipolar plates  18  and  30  are provided between the fuel cells in the fuel cell stack. A hydrogen reactant gas flow from flow channels  28  in the bipolar plate  30  reacts with the catalyst layer  26  to dissociate the hydrogen ions and the electrons. Airflow from flow channels  32  in the bipolar plate  18  reacts with the catalyst layer  22 . The hydrogen ions are able to propagate through the membrane  16  where they electro-chemically react with the oxygen-in the airflow and the return electrons in the catalyst layer  22  to generate water as a by-product. The bipolar plate  18  includes lands  34  between the flow channels  32  and the bipolar plate  30  includes lands  36  between the flow channels  28 . Cooling fluid flow channels  38  are provided in the bipolar plate  18  and cooling fluid flow channels  40  are provided in the bipolar plate  30 .  
         [0023]     The following discussion of the invention is directed to a process for determining the variability of the cells in a fuel cell stack. Particularly, the invention includes determining the difference between the voltage outputs of the fuel cells in a fuel cell stack based on water accumulation in the cathode side flow field channels to determine how often the cathode airflow needs to be pulsed to remove the water from the channels. When the cathode side of a fuel cell is pulsed with an increased airflow, the cell voltage responds as a result of an increase in the partial pressure of oxygen, a reduction in partial pressure of water, an increase in pressure due to an increased pressure drop and higher flows, and an increase in resistance as a result of the MEA drying. The first three factors result in an increase in cell voltage, but the fourth factor results in a decrease in-cell voltage. The overall stack voltage is a super-position of the four factors. Thus, the cell that has the higher water accumulation will take the longest to dry out, and the reduction in the voltage will be delayed or may not even show. Therefore, cells having different water accumulation show different voltage patterns in response to a cathode flow pulse.  
         [0024]     The present invention proposes detecting cell-to-cell variation in water buffers by looking at individual voltage patterns of the cells. This can be a basis of an online diagnostic that can be used to trigger remedial action to address low power stability. For example, the primary remedial action to address low performing cells, low power and stability is to perform cathode flow pulsing. However, currently there is no diagnostics to detect, trigger or stop the cathode pulsing. The duration and height of the cathode flow pulse would depend on the properties of the diffusion media of the fuel cell. For example, as the water holding capacity of the diffusion media in the MEA decreases, the cathode flow pulse can have a lower amplitude and duration.  
         [0025]      FIG. 2  is a flow chart diagram  50  showing one operation for providing cathode airflow pulses to remove water accumulated in the cathode side flow field to increase stack stability, according to an embodiment of the present invention. The algorithm first determines whether the stack is in a low power output or in a sustained idle operation for a predetermined period of time at decision diamond  52 . The particular low power level and the time are application specific for different fuel cell stack designs. In one non-limiting embodiment, the stack must be outputting a stack power of about 0.2 A/cm 2  or less continuously for about five minutes or more. These values are application specific and would vary for different stacks. If the stack is in the sustained idle condition at the decision diamond  52 , then the algorithm provides a cathode pulse airflow to excite stack stability voltage patterns at box  54 . In one non-limiting embodiment, the cathode airflow is pulsed to a level that would provide about half the total output power of the stack for about twenty seconds, i.e. the pulse amplitude and duration.  
         [0026]     The algorithm then performs online analysis of the voltage pattern of the output of each cell to detect the extent of cell-to-cell output voltage variations at box  56 .  FIG. 3  is a graph with time on the horizontal axis and cell voltage on the vertical axis showing a typical or representative pattern exhibited by cell voltages in a stack that has a large cell-to-cell variation in hydration, and is indicative of a stack that would have low power instability and low performing cells. Particularly, the graph includes a plurality of graph lines  60  where each graph line  60  is the output voltage of one fuel cell in the stack. A cathode airflow pulse is initiated at line  62  and ends at line  64 . As is apparent, the output voltages of the cells between the lines  62  and  64  are significantly different for some of the cells indicating a large cell-to-cell variation in output voltage. In other words, some of the cells have a much higher water accumulation in the cathode flow channels than other cells.  
         [0027]      FIG. 4  shows the output voltage pattern for the cells in the stack that has been pulsed a certain number of times so that most of the water has been removed from the cathode flow channels in all of the cells, indicating a stack having low cell variability. The cell output voltages have a similar voltage pattern indicating a small cell-to-cell variation and stack stability. It should be noted that  FIGS. 3 and 4  are not to sale, and are provided to show representative differences in the cell voltage dynamic pattern for stable and unstable stacks.  
         [0028]     The algorithm looks at the voltage patterns of the cells to determine the cell-to-cell variation at box  56 , and how often the cathode airflow should be pulsed during the idle condition. The algorithm can use any suitable technique for determining the difference between the cell voltages during the pulse, as would be appreciated by those skilled in the art.  
         [0029]     The algorithm then determines how often the cathode airflow pulses will be provided to achieve a cell voltage pattern similar to that shown in  FIG. 4  at box  58 . For example, if the algorithm determines the variability between the cell output voltages is of the type shown in  FIG. 3 , then the algorithm may provide a command to pulse the cathode airflow every three minutes, during the idle period when the output power from the stack  12  is low. The voltage pattern is observed during each pulse. If this time frame of airflow pulsing eventually generates a pattern of cell output voltages of the type shown in  FIG. 4 , the algorithm may control the airflow pulses to be on the order of every twenty minutes, for example, or some other suitable time frame. The pulse amplitude and duration can also be controlled depending on the cell-to-cell variability. Thus, the cathode airflow is not pulsed more often than it&#39;s necessary to maintain stack stability, and thus the drying of the membranes will be minimized as a result of the cathode airflow. The values of three minutes and twenty minutes are only representative examples, and would vary from stack to stack.  
         [0030]     The graph lines in  FIGS. 3 and 4  show that different cells with different amounts of water accumulation will have different dynamic patterns in response to a cathode flow pulse. The initial peak in the cell voltages during pulsing of the cathode airflow corresponds to the increase in partial pressure of the air in the fuel cell. The dip in the cell voltage during the pulse is caused by an increase in membrane resistance as it dries out. Therefore, if a cell is flooded and has a lot of water uptake in its diffusion media layer, a cathode pulse may only dry a fraction of the water in the diffusion media layer and not dry the membrane. Consequently, that cell will not show the dip in voltages associated with the increase in membrane resistance. Therefore, the stack is maintained stable at low power outputs, without drying the stack membranes more than necessary.  
         [0031]     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.