METHOD FOR DETERMINING A NORMALIZED HIGH-FREQUENCY RESISTANCE OF A FUEL CELL STACK

Abstract

Method for determining a normalized high-frequency resistance of a fuel cell stack, wherein the method comprises: the high-frequency resistance of the fuel cell stack is measured; After measuring the high-frequency resistance of the fuel cell stack, the relative humidity of the cathode air entering the fuel cell stack is increased; The fuel cell stack is intermittently drained to improve water regulation within the fuel cell stack; the high-frequency resistance of the fuel cell stack is measured after humidification; and The high-frequency resistance of the fuel cell stack measured after humidification is subtracted from the high-frequency resistance of the fuel cell stack measured before humidification, and a high-frequency resistance of a reference fuel cell stack is added to determine the normalized high-frequency resistance of the fuel cell stack.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] This invention relates to a method for determining a normalized high-frequency resistance of a fuel cell stack. 2. Discussion of related techniques

[0002] Hydrogen is a very attractive fuel because it is pure and can be used to efficiently generate electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device consisting of an anode and a cathode with an electrolyte in between. The anode takes in hydrogen gas, and the cathode takes in oxygen or air. The hydrogen gas is split at the anode catalyst to produce free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and electrons at the cathode catalyst to produce water. The electrons from the anode cannot pass through the electrolyte and are therefore carried through a load where they perform work before being delivered to the cathode.

[0003] Proton exchange membrane fuel cells (PEMFCs) are a popular fuel cell for vehicles. A PEMFC generally features a proton-conducting solid polymer electrolyte membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically contain finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is applied to opposite sides of the membrane. The combination of the catalytic anode mixture, the catalytic cathode mixture, and the membrane defines a membrane electrode assembly (MEA). Each MEA is usually layered between two layers of porous material, a gas diffusion layer (GDL), which protects the mechanical stability of the membrane and promotes uniform reactant and moisture distribution.The part of the MEA that separates the anode and cathode flows is called the active region, and only in this region can water vapor be freely exchanged between the anode and cathode. MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0004] Typically, multiple fuel cells are combined in a fuel cell stack to generate the desired power output. For example, a typical fuel cell stack for a vehicle might contain two hundred or more stacked fuel cells. The fuel cell stack receives a cathode inlet gas, typically an airflow, which is forced through the stack by a compressor. Not all of the oxygen is consumed by the stack, and some of the air is discharged as a cathode exhaust, which may contain water as a stack byproduct. The fuel cell stack also receives an anode hydrogen inlet gas, which flows into the anode side of the stack.

[0005] A fuel cell stack comprises a series of bipolar plates (separators) positioned between the various MEAs in the stack, with the bipolar plates and the MEAs positioned between two end plates. The bipolar plates feature 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, allowing the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates, allowing the cathode reactant gas to flow to the respective MEA. One end plate has anode gas flow channels, and the other end plate has 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 have flow channels through which a cooling fluid flows.

[0006] Fuel cell membranes are known to absorb water, which is necessary to provide proton conductivity. However, the water absorption behavior of fuel cell membranes causes an increase in membrane volume when conditions become more humid or wet, and a decrease in volume when conditions become drier. Changes in cell membrane volume can cause mechanical stress on the membrane itself and adjacent fuel cell components. Additionally, an excessively moist membrane can cause problems during low-temperature environments, where freezing of the water within the fuel cell stack can create ice that blocks flow channels and impairs system restart.Membranes that are too dry may have insufficient electrical conductivity at the next system restart, which can impair restart performance and reduce stack durability.

[0007] In engineering, it is known to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust of a fuel cell stack and use this water to humidify the cathode inlet airflow. It is also known to use relative humidity (RH) sensors to monitor the humidification of the cathode inlet airflow. However, RH sensors can be expensive and unreliable. Therefore, there is a need to provide a method for maintaining an appropriate level of cell membrane humidification without relying on RH sensors.

[0008] The high-frequency resistance (HFR) is a well-known property of fuel cells and is closely related to the ohmic resistance, or membrane proton resistance, of fuel cell membranes. The ohmic resistance itself is a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes in a fuel cell stack within a specific band of excitation current frequencies, the degree of fuel cell membrane humidification can be determined. This HFR measurement also allows for an independent measurement of fuel cell membrane humidification, thus eliminating the need for RH sensors. However, variations in the 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 construction or compression.

[0009] Furthermore, variations in HFR measurements can also occur due to deterioration of stack components over the fuel cell stack's lifetime, such as delamination, resulting in an increase in contact resistance. These variations in HFR measurements can be cumulatively referred to as non-membrane contact resistances. Non-membrane contact resistances are considered "noise" and can lead to inaccuracies in determining cell membrane humidification. The non-membrane contact resistance can be quite large, e.g., 20 milliohm-cm. 2 .

[0010] Accordingly, there is a need in engineering to calculate how much of the HFR measurement is caused by various non-membrane contact resistances, in order to enable the fuel cell stack control system to remove the HFR caused by non-membrane contact resistances, in order to determine how much of the total HFR measurement is caused by the proton resistance of the fuel cell membrane, also known as the basic high-frequency resistance of the fuel cell stack.

[0011] Conventional methods for determining the high-frequency resistance of a fuel cell stack are known from documents DE 10 2010 051 220 A1, DE 10 2010 046 149 A1, DE 10 2009 023 882 A1, DE 10 2008 047 389 A1 and DE 10 2004 042 780 A1, whereby documents DE 10 2010 051 220 A1 and DE 10 2010 046 149 A1 were only published after the priority date of the present application. SUMMARY OF THE INVENTION

[0012] A method according to the invention for determining a normalized high-frequency resistance of a fuel cell stack comprises measuring the high-frequency resistance of the fuel cell stack; increasing the relative humidity of the cathode air entering the fuel cell stack after measuring the high-frequency resistance of the fuel cell stack; intermittently draining the fuel cell stack to improve water regulation in the fuel cell stack; and measuring the high-frequency resistance of the fuel cell stack after humidification.and the high-frequency resistance of the fuel cell stack measured after humidification is subtracted from the high-frequency resistance of the fuel cell stack measured before humidification, and a high-frequency resistance of a reference fuel cell stack is added to determine the normalized high-frequency resistance of the fuel cell stack.

[0013] Additional features of the present invention will become apparent from the following description and the attached claims in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of a fuel cell system; Fig. Figures 2a-2c are recorded graphs of the HFR of fifty fuel cell stacks operated under dry, humid, and normalized conditions, respectively; and Fig. Figure 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 flow densities. DETAILED DESCRIPTION OF THE EXECUTION FORMS

[0014] The following discussion of embodiments of the invention, which relates to a method for determining a basic stack resistance for high-frequency resistance-based control of relative humidity, is merely exemplary.

[0015] Fig. Figure 1 is a schematic block diagram of a fuel cell system 10, which includes a fuel cell stack 12. The fuel cell stack 12 receives hydrogen from a hydrogen source 14 on an anode inlet line 16 and delivers an anode exhaust on line 18. A drain valve 26 is provided in the anode exhaust line 18 to allow anode exhaust to be expelled as needed. A compressor 20 supplies an airflow to the cathode side of the fuel cell stack 12 on a cathode inlet line 22 through a water vapor transfer (WVT) unit 24, which humidifies the cathode inlet air. A cathode exhaust is discharged from the stack 12 on a cathode exhaust line 28. The exhaust line 28 directs the cathode exhaust to the WVT unit 24 to provide the moisture for humidifying the cathode inlet air.A bypass line 30 is provided around the WVT unit 24 to allow the cathode exhaust gas to bypass the WVT unit 24. A bypass valve 32 is provided in the bypass line 30 and is controlled to selectively divert the cathode exhaust gas through and around the WVT unit 24 to provide the desired amount of moisture for the cathode inlet air.

[0016] 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 the membrane proton resistance of the fuel cell stack 12. The membrane proton resistance is a function of the membrane humidification of the fuel cell stack 12; however, variations in design parameters, such as compression and variability of parts, degradation of fuel cell stack components over the lifetime of the stack 12, and measurement errors by the HFR sensor 36 itself can cause variations in the HFR sensor measurements from stack to stack.Therefore, HFR-based membrane humidification control systems must be able to adapt to these variations to ensure that the humidification level of the fuel cell membrane is maintained at a suitable level for the lifetime of the fuel cell stack 12.

[0017] 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 bypass valve 32 and the drain valve 26.

[0018] Variations in components of the rest of the system can also affect fuel cell stack humidification. For example, consider a vehicle with an inefficient water vapor transmission unit (WVT) and a fuel cell stack with a low high-friction (HFR) sensor reading. This fuel cell stack operates drier than desired, and the HFR sensor fails to detect this. Alternatively, consider another vehicle with a highly efficient WVT and a fuel cell stack with a high HFR sensor reading. This fuel cell stack operates more humid than desired, and the HFR sensor fails to detect this. Additionally, other system components, such as valves, thermocouples, heat exchangers, pressure and delta-p transducers, etc., can also cause variations in HFR sensor readings.

[0019] The Fig. Figures 2a-2c are recorded graphs of the HFR measurements of fifty fuel cell stacks operated under dry conditions, humid conditions, and normalized conditions, respectively. Fig. Figures 2a-2c show that the variation narrows when the actual HFR is normalized to the stack HFR measurement under humid conditions. When operating under dry and humid conditions, the stacks exhibit a variation in HFR measurements that can be quite large, as shown in the Fig. 2a and Fig. 2b is shown. Fig. Figure 3 is a graph with RH on the x-axis and HFR on the y-axis, showing that as the cell membrane humidification level approaches 100%, the HFR measurement of the cell membrane converges to a known value, regardless of the variation in HFR sensor measurements that may occur at lower RH values. Thus, the variation in HFR of the stack of Fig. 2, which operate under dry conditions, are eliminated because the stacks converge to a known value at 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 at membrane saturation is shown in the normalized plotted graph of Fig. 2 shown. Accordingly, a membrane proton resistance can be illustrated using the following equation: HFR=HFRbas+f(RH)

[0020] Where HFR base is the base stack resistance of the fuel cell membrane and f(RH) is the resistance increase of the fuel cell membrane as a function of the relative humidity.

[0021] Back to Fig.Figure 2 shows the fuel cell stacks in the normalized recorded diagram, that once an HFR “noise” is determined and subtracted for each fuel cell stack, as detailed below, the remaining membrane resistance for each of the fuel cell stacks is similar, for example, about 90 milliohm-cm. 2 Thus, by removing HFR “noise” due to various contact resistances and other non-membrane resistances, a fuel cell membrane humidification can be calculated for each fuel cell stack with a reasonable degree of accuracy. How the HFR “noise” is determined and filtered out is described in detail below.

[0022] One way to account for stack-to-stack variation in HFR measurement due to HFR “noise” is to determine each membrane proton resistance of the fuel cell stack by measuring the base resistance (j) under humidified conditions. (HFR law) to determine and normalize this against the HFR measurement of a reference fuel cell stack, such as a fuel cell stack known to have accurate compression and thus no noticeable contact resistance (i.e., a stack without “noise”). To determine a base stack resistance under humidified conditions or a membrane proton resistance, the following options are available: (1) Ensure that the stack has the base stack resistance under humidified conditions as determined during handover to the module, i.e.,(1) before installation in a vehicle, (2) determining a base stack resistance under humidified conditions by wet operation of the stack, wherein the controller 34 learns this itself before the fuel cell stack is installed in a vehicle (for example, in certain situations modules undergo a "break-in" hydration step, which allows self-learning at this step to be combined with the value stored in the memory of the controller 34), or (3) allowing a base stack resistance to be learned by the controller 34 under humidified conditions after the fuel cell stack 12 has been installed in place in a vehicle and is determined during a cold and wet commissioning.If, in addition, a contact resistance changes significantly over the lifetime of the fuel cell stack 12, it may become necessary to periodically determine a base stack resistance under humidified conditions while a vehicle is undergoing a cold and wet start.

[0023] Once it is determined that the base resistance of the fuel cell stack must be measured under humid conditions, which may occur periodically over the lifetime of the fuel cell stack 12, the first step for the controller 34 is to determine the temperature of the fuel cell stack 12 at startup. This ensures that the stack 12 is not hot, as saturation of the stack membrane is part of the self-learning process and is more easily achieved using a cold and humid start. Calibration and implementation may determine the use of a trigger based on ambient temperature, thus eliminating the need for active diagnostics.It is readily apparent to the person skilled in the art that various methods for determining that the temperature is suitable for implementing the determination of the base resistance of the fuel cell stack under humid conditions are available without deviation from the scope of protection of the present invention.

[0024] Once the controller 34 determines that the fuel cell stack 12 is cold upon power-up, the level of relative humidity discharged from the fuel cell stack 12 onto the cathode discharge line 28 is set to greater than or equal to 100%. The base stoichiometry setpoint is also increased, with the base stoichiometry potentially being twice the nominal stoichiometry or higher at low stack power. The inlet RH at the cathode air inlet line 22 is set to approximately 80% to fully utilize the WVT unit 24. Increasing the stoichiometry lowers the temperature setpoint to achieve high cell membrane humidification of the fuel cell stack 12 and also reduces the likelihood of flooding in the anode and cathode flow channels of the fuel cell stack 12. The determination of the hydration state of the membrane electrode assembly is described in detail below.

[0025] Once the cathode inlet relative humidity (RH) on the cathode air inlet line 22 and the outlet RH on the cathode exhaust line 28 are set to the desired values, intermittent draining of the fuel cell stack 12 occurs, preferably by draining to emissions, to maximize the drain rate and improve anode water management. The increased stoichiometry serves to lower the temperature setpoint, as discussed above, and the full opening of any cooler bypass valve provides maximum cooling. The length of time the fuel cell stack 12 is maintained at an outlet RH greater than or equal to 100% depends on the cooling capacity of the fuel cell system 10. Therefore, the self-learning of the base resistance of the fuel cell stack 12 may be more effective under cold conditions than under warm conditions, as saturation is more readily achieved when the fuel cell stack 12 is cold.

[0026] A water buffer model in the controller 34 integrates the product water and estimates the hydration state of the fuel cell membrane electrode assembly (MEA) and the diffusion medium of the stack 12. When the model estimates that the MEA is near saturation, which is a calibratable estimate, the self-learning process of the controller 34 is triggered. During self-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 a few seconds. This data is then used as HFRjwet filtered and then stored in non-volatile memory of controller 34. During the subsequent operating sequence until the next HFRjwet -Update: The measurements of the HFR sensor 36 are updated by the controller 34 by comparing the measurement of HFRjwet normalized using the HFR measurement of a base resistance of a reference fuel cell stack under humidified conditions and using the following equation: HFRjnorm(t)=HFRjraw(t)−HFRjwer+HFRrefwet

[0027] Where HFRjnorm (t) is the normalized HFR HFRjraw (t) the measured HFR including “noise”, HFRjwet the HFR is, after the membrane of the fuel cell stack has been completely saturated (i.e., the HFR is a measure of the membrane proton resistance) and HFRrefwet is the HFR of the reference stack, which operates with a fully humidified membrane. Thus, the key to HFR-based RH control using the controller 34 is not feedback to the absolute value of the HFR, but rather measuring the HFR in reference to a baseline HFR of the fuel cell stack during a humidified state, which is then compared to the HFR of a reference stack baseline HFR during a humidified state.

[0028] A reconditioning mode of the fuel cell stack 12 can be used to generate the humid conditions necessary for the self-learning of the controller 34, thus replacing the steps leading to the self-learning of the controller 34 as described above, since the fuel cell stack membranes are fully saturated during the reconditioning mode. Additionally, the periodic use of this strategy throughout the lifetime of the fuel cell stack 12 can enable the controller 34 to detect and filter out changes in relative humidity (RH) relative to the high-temperature fructose (HFR) over the lifetime of the fuel cell stack 12, or changes in the operation of the rest of the system over time, such as water vapor transfer efficiency, bypass valve operation, etc.

Claims

[1] A method for determining a normalized high-frequency resistance of a fuel cell stack, the method comprising: the high-frequency resistance of the fuel cell stack is measured; after measuring the high-frequency resistance of the fuel cell stack, the relative humidity of cathode air entering the fuel cell stack is increased; the fuel cell stack is intermittently drained to improve water regulation in the fuel cell stack; the high-frequency resistance of the fuel cell stack is measured after humidification; and the high-frequency resistance of the fuel cell stack measured after humidification is subtracted from the high-frequency resistance of the fuel cell stack measured before humidification and a high-frequency resistance of a reference fuel cell stack is added to determine the normalized high-frequency resistance of the fuel cell stack.

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