Abstract:
A system and method for breaking-in and humidifying membrane-electrode-assemblies (MEAs) in a fuel cell stack. The method includes performing voltage cycling and humidification of the MEAs in the stack, including one or more temperature steps wherein current density of the stack is cycled within a predetermined range for each of the one or more temperature steps. The method also includes maintaining a fuel cell stack voltage within a predetermined range, and maintaining anode and cathode reactant flows at an approximate set-point during the current density cycling of the one or more temperature steps to break-in and humidify the MEAs in the stack so that the stack is able to operate at a predetermined threshold for a fuel cell stack voltage output capability.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method for membrane-electrode-assembly (MEA) break-in and voltage recovery and, more particularly, to a method for MEA break-in and voltage recovery that includes increasing a fuel cell stack temperature in a stepwise manner from approximately room temperature to a temperature that is consistent with high fuel cell stack load operation, maintaining constant anode and cathode reactant flows for each temperature step and cycling fuel cell stack current density over a number of cycles to provide an efficient and fast break-in for the MEAs in the fuel cell stack. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical 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 catalytically split in an oxidation half-cell reaction in the anode catalyst layer to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen 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. 
         [0005]    Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type 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 dispersed 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 where it forms the anode and cathode catalytic layers. The combination of the anode catalytic layer, the cathode catalytic layer and the membrane define a membrane electrode assembly (MEA). MEAs require adequate fuel and oxidant supply and also humidification for effective operation. 
         [0006]    Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input 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 input gas that flows into the anode side of the stack. 
         [0007]    A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
         [0008]    The MEA within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane and anode and cathode catalytic layers is low enough to effectively conduct protons. Humidification for the membrane and the ionomer in the catalytic layers may come from the stack water by-product or external humidification. The MEAs in a newly built fuel cell stack are dry, i.e., they essentially lack ionic conductivity. 
         [0009]    As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will typically include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements. 
         [0010]    Both break-in or conditioning and voltage recovery are required for the MEAs in a newly fabricated fuel cell stack to obtain optimal performance during initial operation of the stack. There are three main functions of MEA break-in and voltage recovery: humidification, removal of residual solvents and other impurities from MEA manufacturing and removal of anions from the catalyst to activate reaction sites. The current state of the art procedures for break-in and voltage recovery of the MEAs require from 1 to over 15 hours of fuel cell operation targeting different levels of resulting functionality. Thus, there is a need in the art to provide a method of break-in and voltage recovery of the MEAs in a short period of time that is still capable of achieving the three main functions and that also provides the targeted performance level. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with the teachings of the present invention, a system and method is disclosed for breaking-in and humidifying membrane-electrode-assemblies (MEAs) in a fuel cell stack. The method includes performing voltage cycling and humidification of the MEAs in the fuel cell stack, including one or more temperature steps wherein current density of the stack is cycled within a predetermined range for each of the one or more temperature steps. The method also includes maintaining a fuel cell stack voltage within a predetermined range, and maintaining anode and cathode reactant flows at an approximate set-point during the current density cycling of the one or more temperature steps to break-in and humidify the MEAs in the stack so that the stack is able to operate at a predetermined threshold for a fuel cell stack voltage output capability. 
         [0012]    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 
         [0013]      FIG. 1  is a cross-sectional view of a fuel cell; 
           [0014]      FIG. 2  is a simplified block diagram of a fuel cell system; 
           [0015]      FIG. 3  is a flow chart diagram of an algorithm for membrane-electrode-assembly (MEA) break-in and voltage recovery; 
           [0016]      FIG. 4  is a graph with time on the horizontal axis, average coolant temperature on the right vertical axis and current density on the left vertical axis that illustrates a procedure for MEA break-in and voltage recovery; and 
           [0017]      FIG. 5  is a graph with time on the horizontal axis, average fuel cell stack voltage on the right vertical axis and stack current density on the left horizontal axis that illustrates a procedure for MEA break-in and voltage recovery. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    The following discussion of the embodiments of the invention directed to a method for membrane-electrode assembly (MEA) break-in and voltage recovery is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0019]      FIG. 1  is a cross-sectional view of a fuel cell  10  that is part of a fuel cell stack of the type discussed below. The fuel cell  10  includes a cathode side  12  and an anode side  14  separated by a perfluorosulfonic acid 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  18 . The diffusion media layers  20  and  24  are porous layers that provide for input gas transport to and water transport from the MEA  18 . A cathode side flow field plate or bipolar plate  28  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 . Coolant flow channels  32  are provided in the bipolar plates  28  and  30  to allow for a cooling fluid to flow through the fuel cells  10 . 
         [0020]      FIG. 2  is a simplified block diagram of a fuel cell system  40  including a fuel cell stack  42 . The fuel cell system  40  is intended to generally represent any type of testing environment for the fuel cell stack  42 . Hydrogen gas from a hydrogen source  44  is provided to the anode side of the fuel cell stack on line  46 . Anode exhaust exits the stack  42  on line  50 . A compressor  62  provides a cathode inlet airflow on a cathode input line  64  to the stack  42 . A cathode exhaust gas is output from the stack  42  on a cathode exhaust line  66 . 
         [0021]    The fuel cell system  40  also includes a thermal sub-system for controlling the temperature of the fuel cell stack  42 . Particularly, a cooling fluid pump  58  pumps a cooling fluid through a coolant loop  56  outside of the fuel cell stack  42  and through the coolant channels  32  in the stack  42 . A radiator  70  and a heater  52  on the coolant loop  56  are used to maintain the stack  42  at a desired temperature, as discussed in more detail below. 
         [0022]    A temperature sensor  60  measures the temperature of the coolant at a coolant inlet to the fuel cell stack  42  and a temperature sensor  68  measures the temperature of the coolant at a coolant outlet of the stack  42 . A voltage sensor  48  measures the average fuel cell voltage for the fuel cells  10  in the stack  42 . A controller  54  receives a temperature signal from the temperature sensor  60  that indicates the temperature of the coolant at a coolant inlet of the stack  42 , and the controller  54  receives a temperature signal from the temperature signal  68  that indicates the temperature of the coolant at a coolant outlet of the stack  42 . The controller also receives a fuel cell voltage signal from the voltage sensor  48  and controls the radiator  70 , coolant pump  58  and heater  52  of the thermal sub-system. The controller  54  further controls the speed of the compressor  62  and the flow of hydrogen from the hydrogen source  44 . An algorithm of the controller  54  performs break-in and voltage recovery for the MEAs in the stack  42  as is discussed in more detail below. The controller  54  may be any control system or central processing unit (CPU), as is readily understood by those skilled in the art. 
         [0023]    As discussed above, both break-in and voltage recovery are required for MEAs in a newly fabricated fuel cell stack to obtain desired performance during initial operation of the stack. The three main functions of MEA break-in and voltage recovery are: humidification, removal of residual solvents and other impurities from MEA manufacturing, and removal of anions from the catalyst to activate reaction sites in order to decrease the overpotentials of the anode and cathode catalytic layers. As is also discussed above, there is a need for a shorter break-in and voltage recovery time of MEAs to reduce the costs associated with the break-in and voltage recovery of the MEAs. By reducing the amount of time needed, the costs associated with production and the usage of reactants such as hydrogen may be reduced. Procedures are currently used in the art mostly employ two separate protocols for voltage recovery and break-in of MEAs, which require a total time in the range between from approximately 1 hour to over 15 hours of fuel cell operation and procedures that are currently used do not have a well defined target. Moreover, the shortest of the procedures currently used that also includes voltage cycling apply a current to the cells in the stack without any relation to cell temperature and/or level of humidification. In the procedure discussed below, the magnitude of the current applied is not increased before a certain predetermined level of humidification is achieved. 
         [0024]      FIG. 3  is a flow diagram  80  of an algorithm for a shorter break-in and voltage recovery of the MEAs  18  in the fuel cell stack  42 . The algorithm is part of the controller  54  shown in  FIG. 2 . The steps of voltage recovery and break-in of the MEAs  18  are combined according to the algorithm into a single sequence that, for this example, requires only approximately 90 minutes to verify that the MEAs  18  in the stack  42  have achieved a targeted performance level capability. Although not shown for the sake of clarity, a shorting check is performed on the fuel cell stack  42  to determine if the fuel cell stack is ready for the algorithm of flow diagram  80  to begin. 
         [0025]    Voltage cycling combined with humidification begins at box  82 . Voltage cycling combined with humidification is performed at several different temperature steps ranging from approximately room temperature to a temperature that is consistent with high load operation. For example, six temperature steps can be used.  FIG. 3  shows three temperature steps for the sake of brevity. As shown in the flow diagram  80 , the first temperature step at box  82  uses a temperature range that is near room temperature, the second temperature step at box  86  uses a temperature range that is above room temperature, and the last temperature step at box  90  uses a temperature range that is consistent with an expected temperature for a stack operating with a high fuel cell stack load. The temperature of a stack operating with a high fuel cell stack load will vary with stack characteristics. 
         [0026]    At each temperature step of the boxes  82 ,  86  and  90 , the voltage cycling is achieved by cycling the load, i.e., cycling the current density, between predetermined lower and upper levels assigned for the load increment for each temperature step. At each load increment, the anode reactant gas flow and the cathode reactant gas flow through the stack  42  are at a constant rate. For example, the anode reactant flow is set such that the anode stoichiometry is approximately 1.5 and the cathode stoichiometry is approximately 1.1 relative to the upper level of the current load increment, i.e., the air flow is kept at a level that is close to the minimum amount of air flow necessary to support the electrochemical reaction. 
         [0027]    For each temperature step of the boxes  82 ,  86  and  90  the range of current density cycling is changed. By way of example, at the box  82  the current density range is approximately 0.1-0.2 A/cm 2 . Voltage cycling at the box  82  continues until the maximum voltage for each current density cycle within the given temperature step of the box  82  does not continue to increase, as determined at decision diamond  84 . If the voltage for each current density cycle for the temperature step of the box  82  does not continue to increase at the decision diamond  84 , the algorithm moves on to the next temperature step and the next current cycling range at the box  86 . For example, the next current density range for the cycle at the box  86  is at a current density range of approximately 0.2-0.3 A/cm 2 . The thermal sub-system and the controller  54 , discussed above, control the stepwise increase in temperature of the fuel cell stack  42 . If the maximum voltage for the current density cycle of the temperature step at the box  82  is still increasing, as determined at the decision diamond  84 , the algorithm returns to the box  82  and continues voltage cycling at the temperature step of the box  82 . 
         [0028]    Once the algorithm moves on to the temperature step at the box  86 , voltage cycling at the box  86  continues until the maximum voltage for each current density cycle within the given temperature step at the box  86  does not continue to increase, as determined at decision diamond  88 . If the voltage is not increasing, the algorithm moves on to the next temperature step and the cycling for the next current density range at box  90 . However, if the maximum voltage for the current density cycling of the temperature step at the box  86  is still increasing, as determined at the decision diamond  88 , the algorithm returns to the box  86  and continues voltage cycling at the temperature step of the box  86 . Once the algorithm moves on to the temperature step at the box  90 , voltage cycling at the box  90  continues until the maximum voltage for each current density cycle within the given temperature step of the box does not continue to increase, as determined at decision diamond  92 . If the voltage is not increasing, the algorithm moves on to a performance verification step at box  94 , as discussed in detail below. However, if the maximum voltage for the current density cycling of the temperature step at the box  90  is still increasing, as determined at the decision diamond  92 , the algorithm returns to the box  90  and continues voltage cycling at the temperature step of the box  90 . 
         [0029]    Humidification is carried out simultaneously with voltage cycling at the boxes  82 ,  86  and  90  as discussed above.  FIG. 4  is a graph with time on the horizontal axis, average coolant temperature on the right vertical axis and current density on the left vertical axis. Line  102  represents the current density cycles for each temperature step similar to that shown in  FIG. 5 , discussed below, and line  104  represents average coolant temperature for the fuel cells  10  in the stack  42 . As shown in  FIG. 4 , the temperature of the fuel cell stack  42  is kept low at the beginning of voltage cycling and humidification. For example, the temperature of the stack  42  at the beginning of voltage cycling and humidification at the box  82  is approximately room temperature, or approximately 25° C. As current density cycling continues, the temperature of the stack  42  is allowed to increase stepwise, as shown by the coolant temperature line  104 . The purpose of starting the voltage cycling and humidification at a low temperature and increasing the temperature of the stack  42  in a stepwise manner is to ensure that the small amount of by-product water produced by the electrochemical reaction at low current density remains in the form of liquid water, i.e., condensed water, so as to enable the membranes  16  of the stack  42  to become adequately humidified and also to begin washing away solvents and impurities that may be present from fabrication out of the stack  42 . It is more efficient to remove anions and platinum oxide from the MEAs  18  at higher temperature, however, the membranes  16  should be adequately humidified before exposed to high fuel cell stack current and operating temperatures to avoid damaging the membranes  16 . Thus, the temperature is started out low and is increased stepwise in the algorithm of flow diagram  80 , as discussed above. 
         [0030]    For each temperature step and the corresponding current density cycling at the boxes  82 ,  86  and  90 , current density cycling can occur as many times as desired.  FIG. 5  is a graph with time on the horizontal axis, average fuel cell voltage on the right vertical axis and current density on the left vertical axis illustrating how current density is increased according to the algorithm of flow diagram  80 . As shown in  FIG. 5 , the voltage of the stack  42  is kept low during voltage cycling and humidification. For example, the voltage may be maintained between −200 and 800 mV throughout the voltage cycling and humidification procedure. In  FIG. 5 , line  100  represents voltage, which is maintained within a predetermined range, and line  102  represents current density, which is increased in increments in parallel with each temperature step as shown in  FIG. 4 . For each temperature step the current density is cycled approximately five times or more, however,  FIG. 5  is merely an example. The number of temperature steps and the number of current density cycles for each temperature step that occur depend on factors such as the desired conditioning of the fuel cell stack  42  and the change in voltage during the current density cycling of the temperature steps. 
         [0031]    If the predetermined threshold at the very end of the voltage cycling and humidification is a certain voltage output capability, the number of current density cycles performed can be targeted for the desired capability and will depend on stack characteristics. For example, if 70% voltage output capability is the target for the fuel cell stack  42 , meaning the stack  42  will be able to operate at 70% power after the algorithm of flow diagram  80  is complete, then the number of current density cycles required at each temperature step to achieve the goal of 70% power will be needed. As stated above, the number of cycles required to achieve the target of 70% voltage output capability will depend on stack characteristics. Under such circumstances, additional break-in through use of the fuel cell stack  42  will be necessary before the stack  42  can achieve 100% power. 
         [0032]    Anode and cathode stoichiometry are kept relatively low during the voltage cycling and humidification to ensure that the by-product water that is produced during the electrochemical reaction is not lost due to high reactant flow rate, particularly a high cathode flow rate, because high reactant flow may dry out the MEAs of the stack  42 . Another reason to keep the cathode air flow low, i.e., cathode stoichiometry low, is to keep the voltage of the stack  42  low. At higher load levels during the current density cycling it is possible that the voltage may drop to approximately −0.1 volts due to low cathode stoichiometry and high water content. While negative voltage is typically undesirable, it is believed that brief instances of a slight negative voltage caused by oxygen depletion will be tolerated by the fuel cell stack  42 . 
         [0033]    After voltage cycling combined with humidification is completed, as determined by the decision diamond  92 , anode and cathode stoichiometry sensitivity tests are performed, although not shown in flow diagram  80  for the sake of clarity. After completing the last temperature step, i.e., the highest temperature step of the box  90 , a performance verification step is performed at box  94 . A performance test of the fuel cell stack  42  at a predetermined load level for chosen operating conditions is performed at the performance verification step of the box  94 . The performance test is performed to confirm that the expected performance level for the stack  42  has been achieved and break-in of the MEAs is complete. If the performance test at the box  94  meets a predetermined performance level at a decision diamond  96 , the algorithm continues to box  98  and the break-in process is considered complete. If the predetermined performance level has not been achieved, as determined at the decision diamond  96 , then the algorithm returns to the box  90  for a repeat of current density cycling at the step with the highest temperature. 
         [0034]    A hydrogen take-over test is performed after the performance test is complete to confirm the absence of cross-over leaks after completion of the voltage cycling and humidification, however, this step is not shown in flow diagram  80  for the sake of clarity. 
         [0035]    The total time required for the algorithm described above is approximately 80-90 minutes, depending on factors such as time required for temperature transitions, the number of current density cycles for each temperature step, and the additional testing such as the performance test. 
         [0036]    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.