Patent Publication Number: US-9853312-B2

Title: Method for determining membrane protonic resistance of a fuel cell stack

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. Utility application Ser. No. 12/696,745, titled Auto-Learning of Base Stack Resistance for HFR Based RH Control, filed Jan. 29, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to a method for determining the membrane protonic resistance of a fuel cell stack using high frequency resistance and, more particularly, to a method for determining the membrane protonic resistance of a fuel cell stack using high frequency resistance by determining how much of the total high frequency resistance is caused by various non-membrane resistances and then removing those resistances. 
     Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean 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 dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst 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. 
     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). Each MEA is usually sandwiched between two sheets of porous material, a gas diffusion layer (GDL) that protects the mechanical integrity of the membrane and helps in uniform reactant and humidity distribution. The part of the MEA that separates the anode and cathode flows is called the active area, and only in this area the water vapors can be freely exchanged between the anode and cathode. MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. 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. 
     A fuel cell stack includes a series of bipolar plates (separators) 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 anode side and cathode side flow distributors (flow fields) 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. 
     Fuel cell membranes are known to have a water-uptake which is necessary to provide proton conductivity. The water-uptake behavior of fuel cell membranes, however, causes an increase of volume of the membranes if conditions become more humid or wet and a decrease of volume if conditions become dryer. Changes in the volume of the cell membranes may cause mechanical stress on the membrane itself and the adjacent fuel cell components. In addition, a membrane that is too wet may cause problems during low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Membranes that are too dry may have too low of an electrical conductivity at the next system restart that affects restart performance and may reduce stack durability. 
     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 of a fuel cell stack, and use the water to humidify the cathode input airflow. It is also known in the art to use relative humidity (RH) sensors to monitor the humidification of the cathode input airflow. However, RH sensors can be costly and unreliable. Therefore, there is a need in the art to provide a method for maintaining an appropriate level of cell membrane humidification without relying on RH sensors. 
     High frequency resistance (HFR) is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance, of fuel cell membranes. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined. This HFR measurement allows for an independent measurement of the fuel cell membrane humidification, thereby eliminating the need for RH sensors. However, variations in HFR measurements of the cell membranes can occur from stack to stack for various reasons, including HFR measurement errors (bias), variations in stack materials, or variations in stack design or compression. Furthermore, variations in HFR measurements can also occur due to degradation of stack components over the life of the fuel cell stack, such as delamination, which causes an increase in contact resistance. These variations in HFR measurements may be cumulatively referred to as non-membrane contact resistances. Non-membrane contact resistances are considered “noise” and may lead to inaccuracies in determining cell membrane humidification. Non-membrane contact resistance, may be quite large, e.g., 20 miliohms-cm 2 . 
     Accordingly, there is a need in the art to calculate how much of the HFR measurement is caused by various non-membrane contact resistances to enable the control system of the fuel cell stack to remove the HFR that is caused by non-membrane contact resistances to determine how much of the total HFR measurement is caused by the protonic resistance of the fuel cell membrane, also known as the base fuel cell stack high frequency resistance. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method for determining membrane humidification by determining the membrane protonic resistance of a fuel cell stack using high frequency resistance-based relative humidity control in a fuel cell stack is disclosed. The method includes identifying the base resistance of the fuel cell stack at humidified conditions and normalizing the base resistance of the fuel cell stack against the base resistance of a reference fuel cell stack. 
     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 
         FIG. 1  is a schematic block diagram of a fuel cell system; 
         FIGS. 2 a -2 c    are plot diagrams of the HFR of fifty fuel cell stacks operated under dry conditions, wet conditions, and normalized, respectively; and 
         FIG. 3  is a graph with relative humidity on the x-axis and HFR on the y-axis illustrating the relationship between HFR and relative humidity at three different stack current densities. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a method for determining base stack resistance for high frequency resistance-based relative humidity control is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a schematic block diagram of a fuel cell system  10  including a fuel cell stack  12 . The fuel cell stack  12  receives hydrogen from a hydrogen source  14  on anode input line  16  and provides an anode exhaust gas on line  18 . A bleed valve  26  is provided in the anode exhaust gas line  18  to allow anode exhaust gas to be exhausted as desired. A compressor  20  provides an airflow to the cathode side of the fuel cell stack  12  on cathode input line  22  through a water vapor transfer (WVT) unit  24  that humidifies the cathode input air. A cathode exhaust gas is output from the stack  12  on a cathode exhaust gas line  28 . The exhaust gas line  28  directs the cathode exhaust to the WVT unit  24  to provide the humidity to humidify the cathode input air. A by-pass line  30  is provided around the WVT unit  24  to allow the cathode exhaust gas to by-pass the WVT unit  24 . A by-pass valve  32  is provided in the by-pass line  30  and is controlled to selectively redirect the cathode exhaust gas through or around the WVT unit  24  to provide the desired amount of humidity to the cathode input air. 
     An HFR sensor  36  measures the high frequency resistance (HFR) of the fuel cell stack  12  to determine the cell membrane humidification of the fuel cell stack  12 . The HFR sensor  36  operates by measuring the ohmic resistance, or membrane protonic resistance, of the fuel cell stack  12 . Membrane protonic resistance is a function of membrane humidification of the fuel cell stack  12 , however, stack-to-stack variation in the HFR sensor measurements may be caused by differences in design parameters, such as compression and variability of parts, degradation of fuel cell stack components over the life of the stack  12 , and measurement errors from the HFR sensor  36  itself. Therefore, HFR-based membrane humidification control systems must be able to adapt to these variations to ensure that the level of humidification of the fuel cell membrane is maintained at an appropriate level for the life of the fuel cell stack  12 . 
     The system  10  also includes a controller  34  that receives HFR measurement signals from the sensor  36  and controls the speed of the compressor  20 , the injection of hydrogen from the hydrogen source  14 , and the position of the by-pass valve  32  and the bleed valve  26 . 
     Variation in parts in the balance of plant (i.e., rest of plant) may also impact fuel cell stack humidification. For example, suppose one vehicle includes an inefficient WVT unit and a fuel cell stack with a low HFR sensor reading. This fuel cell stack will be running dryer than desired and the HFR sensor will not detect it. Alternatively, suppose another vehicle has a WVT unit with high efficiency and a fuel cell stack with a high HFR sensor reading. This fuel cell stack will be running wetter than desired and the HFR sensor will not detect it. In addition, the rest of plant items, such as valves, thermocouples, heat exchangers, pressure and delta-P transducers, etc., could also cause variations in HFR sensor measurements. 
       FIGS. 2 a -2 c    are plot diagrams of the HFR measurements of fifty fuel cell stacks operated under dry conditions, wet conditions, and normalized, respectively.  FIGS. 2 a -2 c    illustrate that variation tightens when the actual HFR is normalized with respect to stack HFR measurement under wet conditions. When operated under dry conditions and wet conditions, the stacks exhibit a variation in HFR measurements that may be quite large, as is illustrated in  FIGS. 2 a  and 2 b   .  FIG. 3  is a graph with RH on the x-axis and HFR on the y-axis that illustrates that as the level of cell membrane humidification approaches 100%, the HFR measurement of the cell membrane will converge to a known value, regardless of the variation in the HFR sensor measurements that may occur at lower RH values. Thus, for example, the variation in HFR of the stacks of  FIG. 2  operated under dry conditions will be eliminated as the stacks converge to a known value upon cell membrane saturation. Thus, HFR “noise” is eliminated when the fuel cell stacks reach an RH value equal to or greater than 100%. This convergence to a known value upon membrane saturation is illustrated in the normalized plot diagram of  FIG. 2 . Accordingly, membrane protonic resistance can be illustrated using the following equation:
 
 HFR=HFR   base   +f ( RH )  (1)
 
Where HFR base  is fuel cell membrane base stack resistance and f (RH) is resistance rise of the fuel cell membrane as a function of relative humidity.
 
     Going back to  FIG. 2 , the fuel cell stacks in the normalized plot diagram illustrate that once HFR “noise” is determined and subtracted for each fuel cell stack, which is described in detail below, the remaining membrane resistance for each of the fuel cell stacks is similar, e.g., around 90 miliohm-cm 2 . Thus, by removing HFR “noise” due to various contact resistances and other non-membrane resistances, fuel cell membrane humidification for each fuel cell stack can be calculated with a reasonable degree of accuracy. How the HFR “noise” is determined and filtered out is set forth in detail below. 
     One way to address stack-to-stack HFR measurement variation due to HFR “noise” is to identify each fuel cell stack&#39;s membrane protonic resistance by determining base resistance (j) at humidified conditions (HFR j   wet )and normalize it against the HFR measurement of some reference fuel cell stack, such as a fuel cell stack which is known to have accurate compression, and thus no appreciable contact resistance (i.e., a stack without “noise”). To identify base stack resistance at humidified conditions, the following options are available: (1) ensure the fuel cell stack comes with base stack resistance at humidified conditions identified during handover to module, i.e., prior to insertion in a vehicle, (2) identify base stack resistance at humidified conditions by running the stack wet and having the controller  34  auto-learn this prior to inserting the fuel cell stack in a vehicle (e.g., in certain situations, modules go through a “break-in” hydration step, thus auto-learning may be combined in this step with the value being stored in the memory of the controller  34 ), or (3) allow base stack resistance at humidified conditions to be learned by the controller  34  after the fuel cell stack  12  has been place in a vehicle, and determined during a cold and wet start-up. Additionally, if contact resistance changes appreciably over the life of the fuel cell stack  12 , it may be necessary to determine base stack resistance at humidified conditions periodically while in a vehicle during a cold and wet start. 
     Once it is determined that the fuel cell stack&#39;s base resistance at humidified conditions needs to be determined, which may occur periodically throughout the life of the fuel cell stack  12 , the first step is for the controller  34  to determine the temperature of the fuel cell stack  12  at key on to ensure that the stack  12  is not warm, as saturation of the stack membranes is part of the auto-learning process and is more readily achieved utilizing a cold and wet start. Calibration and implementation may dictate having a trigger based on ambient temperature and thus no active diagnostics. It will be readily apparent to those skilled in the art that various methods for determining that the temperature is suitable to implement the determination of the base resistance of the fuel cell stack at humidified conditions are available without departing from the scope of the present invention. 
     Once the controller  34  determines that the fuel cell stack  12  is cold at key on, the level of relative humidity exhausted from the fuel cell stack  12  on the cathode exhaust line  28  is set to greater than or equal to 100%. The base stoichiometry set-point is also increased, where base stoichiometry may be twice as high as nominal stoichiometry, or more if at low stack power. Inlet RH on the cathode air input line  22  is set to approximately 80% to fully utilize the WVT unit  24 . Increasing the stoichiometery lowers the temperature set-point to achieve high cell membrane humidification of the fuel cell stack  12  and also reduces the chance of flooding in the anode and cathode flow channels of the fuel cell stack  12 . Determination of the hydration state of the membrane electrode assembly is discussed in detail below. 
     Once the cathode inlet RH on the cathode air input line  22  and the outlet RH on the cathode exhaust line  28  are set to the desired values, intermittent bleeds of the fuel cell stack  12  occur, preferably by bleeding to emissions, to maximize bleed velocity to improve anode water management. The increased stoichiometry operates to lower the temperature set-point, as discussed above, and fully opening any radiator by-pass valve will provide maximum cooling. The length of time the fuel cell stack  12  is kept at an outlet RH greater than or equal to 100% will depend on the cooling capacity of the fuel cell system  10 . Hence, auto-learning of the base resistance of the fuel cell stack  12  may be more effective in cold conditions rather than warm, as saturation is more readily achieved when the fuel cell stack  12  is cold. 
     A water buffer model in the controller  34  integrates the product water and estimates the fuel cell membrane electrode assembly (MEA) and diffusion media hydration state of the stack  12 . When the model estimates that the MEA is close to saturation, which is a calibratable estimation, the auto-learning operation of the controller  34  is triggered. During auto-learning, the HFR of the fuel cell stack  12  is measured by the controller  34  using the HFR sensor  36  for a period of time, such as for a few seconds. This data is filtered as HFR j   wet  and is then saved in a non-volatile memory of the controller  34 . During subsequent operation, until the next HFR j   wet  update, the HFR sensor  36  measurements are normalized by the controller  34  by comparing the measurement of HFR j   wet , to the HFR measurement of a reference fuel cell stack&#39;s base resistance at humidified conditions, and using the following equation:
 
 HFR   j   norm ( t )= HFR   j   raw ( t )− HFR   j   wet   +HFR   ref   wet   (2)
 
Where HFR j   norm (t) is HFR normalized, HFR j   raw (t) is measured HFR including “noise”, HFR j   wet  is the HFR after the membrane of the fuel cell stack  12  has been fully saturated (i.e., HFR is a measure of membrane protonic resistance), and HFR ref   wet  is the HFR of the reference stack operating with a fully saturated membrane. Thus, the key to HFR based RH control using the controller  34  is not to feedback on the absolute value of HFR, but the HFR measurement in reference to a base HFR of the fuel cell stack during a humidified state, which is then compared to the HFR of a reference stack base HFR during a humidified state.
 
     A reconditioning mode of the fuel cell stack  12  could be used to generate the wet conditions necessary for auto-learning of the controller  34  to take place, and thus would replace the steps leading up to the auto-learning of the controller  34  described above, as the fuel cell stack membranes are fully saturated during the reconditioning mode. In addition, employing this strategy periodically during the life of the fuel cell stack  12  may enable the controller  34  to detect and filter out changing RH versus HFR over the life of the fuel cell stack  12 , or changing operation of the rest of plant over time, for example, water vapor transfer efficiency, bypass valve operation, etc. 
     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.