Intelligent vehicle-integrated reactive recovery strategy

Abstract

A method for determining when to perform a stack voltage recovery process of a fuel cell stack in a fuel cell system, the method comprising: - determining an estimate of an irreversible voltage loss of the fuel cell stack; - Determining an estimate of a current voltage of the fuel cell stack; - Determining a difference between the irreversible voltage loss and the current voltage; - Determining whether the difference between the irreversible voltage loss and the current voltage is greater than a predetermined voltage threshold, and which includes determining whether the difference between the irreversible voltage loss and the current voltage at 100% of the stack power is greater than a first predetermined voltage threshold, and determining whether the difference between the irreversible voltage loss and the current voltage at 25% of the stack power is greater than a second predetermined voltage threshold; and - Performing the stack voltage recovery process when one of the differences between the irreversible voltage loss and the actual voltage loss is greater than a predetermined voltage threshold.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention generally relates to a method for determining when to operate a voltage recovery algorithm for recovering a reverse voltage loss of a fuel cell stack in a fuel cell system, and to a method for determining when to operate a voltage recovery algorithm for recovering a reverse voltage loss of a fuel cell stack in a fuel cell system, the method determining whether a difference between an irreversible voltage loss and a current stack voltage is greater than a predetermined value. 2. Discussion of the relevant technology

[0002] Hydrogen is an attractive fuel because it is clean and because it can be used to effectively generate electricity in a fuel cell. A hydrogen fuel cell is also an electrochemical device that includes an anode and a cathode with an electrolyte in between. The anode receives hydrogen gas, and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to produce free protons and electrons. The protons migrate through the electrolyte to the cathode. The protons react with the oxygen, and the electrons in the cathode catalyst produce water. The electrons from the anode cannot migrate through the electrolyte and are therefore conducted through a load to produce work before being sent to the cathode.

[0003] Proton exchange membrane fuel cells (PEMFCs) are a common fuel cell for vehicles. The PEMFC generally comprises a proton-conducting membrane made of a solid polymer electrolyte, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, comprise finely divided catalytic particles of a highly active catalyst such as platinum (Pt), typically supported on carbon particles and mixed with an ionomer. The catalyst mixture is positioned on opposite sides of the membrane. The combination of the anode-catalyst mixture, the cathode-catalyst mixture, and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0004] Multiple fuel cells are typically combined in a fuel cell stack to produce a desired output. 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, which is typically an air stream forced through the stack by a compressor. Not all of the oxygen is consumed by the stack, and some of the air is exhausted as cathode exhaust, which may contain water as a stack byproduct. The fuel stack also receives an anode hydrogen input gas, which flows into the stack on the anode side.

[0005] A fuel cell stack includes a series of bipolar plates positioned between the different MEAs in the stack, with the bipolar plates and MEAs positioned between two endplates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in a stack. Anode gas flow fields are provided on the anode side of the bipolar plates, allowing the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plate, allowing the cathode reactant gas to flow to the corresponding MEA. One endplate includes anode gas flow channels, and the other endplate includes cathode gas flow channels. The bipolar plates and endplates 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 contain flow channels through which a cooling fluid flows.

[0006] The membrane within the fuel cell must have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification can come from the stack water byproduct or from external humidification. The flow of reactants through the stack flow channels has a drying effect on the cell membranes, which is most pronounced at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels can prevent the reactants from flowing through and can cause cell failure due to the low reactant gas flow, which stresses cell stability. The accumulation of water in the reactant gas flow channels as well as within the gas diffusion layer (GDL) is particularly problematic at low stack output loads.

[0007] As mentioned above, water is generated as a byproduct of stack operation. Therefore, the cathode exhaust 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 water in the cathode exhaust and use the water to humidify the cathode inlet air stream. Water in the cathode exhaust on one side of the water transfer elements, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream on the other side of the water transfer element.

[0008] There are a number of mechanisms that occur during the operation of a fuel cell system that cause a permanent loss of stack voltage and performance, such as loss of catalyst activity, corrosion of the catalyst support, or pinhole formation in the cell membranes. However, there are other mechanisms that can cause stack voltage losses that are essentially reversible, such as drying out of the cell membranes, oxide formation of the catalyst, and contaminants deposited on both the anode and cathode sides of the stack.

[0009] To achieve commercial viability for a PEM fuel cell system, it is generally necessary to limit the precious metal content, namely platinum or platinum alloy catalysts, on the fuel cell electrodes to reduce the overall cost of the system. As a result, the total available electrochemically active surface area of ​​the catalyst can be limited or reduced, making the electrodes more susceptible to contamination. The source of contamination that can cause stack voltage loss can come from the anode and cathode reactant gas supply streams, including the humidifying water, or be generated within the fuel cells due to degradation of the MEA, stack seals, and / or bipolar plates. One particular type of contamination includes negatively charged anions, such as chlorine, or sulfates, such as SO4. 2-The anions tend to adsorb onto the platinum catalyst surface of the electrode during normal fuel cell operation when the cathode potential is typically above 650 mV, blocking the active site for oxygen reduction reactions and leading to cell voltage losses. Furthermore, when proton conductivity is highly dependent on a contamination-free platinum surface, such as nanostructured thin film (NSTF)-type electrodes, additional losses are caused by the reduced proton conductivity.

[0010] US 845 0 020 B2 discloses a method for determining the performance of the fuel cells in a fuel cell stack, which includes determining a remaining catalyst surface area and a catalyst support area of ​​the catalyst layers in the fuel cells of the fuel cell stack, which can be used to estimate the stack voltage. The method includes determining a total parasitic flux of the fuel cell stack to determine a parasitic cross-current and a short-circuit resistance of the fuel cell stack, thus determining the catalyst surface area. By knowing the catalytic surface area, the voltage magnitude that the stack should provide can be determined. Another method for reconditioning is disclosed in DE 10 2010 048 254 A1.In addition, DE 11 2007 002 792 T5 describes a cell voltage regeneration treatment of a fuel cell stack. SUMMARY OF THE INVENTION

[0011] According to the teachings of the present invention, a method is disclosed for determining when to operate a voltage recovery process for recovering a reversible voltage loss of a fuel cell stack in a fuel cell system. The method includes estimating an irreversible voltage loss of the fuel cell and a current voltage of the fuel cell stack, and determining whether a difference between the estimated irreversible voltage loss and the estimated current voltage exceeds a threshold. If so, the voltage recovery process is performed.

[0012] Additional features of the present invention will become more apparent from the following description and the 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; and Fig. 2 is a flowchart showing an operation for determining when to execute a voltage recovery algorithm for recovering a reversible voltage loss of the fuel cell stack in the fuel cell system shown in Fig. 1 is to be carried out. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] The following discussion of embodiments of the invention relating to a method for determining when to operate a voltage recovery algorithm to recover a reversible voltage loss of a fuel cell stack in a fuel cell system is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses.

[0014] Fig. 1 is a schematic block diagram of a fuel cell system 10 including a fuel cell stack 12, the system 10 being capable of providing the stack operating conditions of cell voltage loss recovery referred to below. A compressor 16 delivers airflow to the cathode side of the fuel cell stack 12 on a cathode input line 14 via a water vapor transfer (WVT) unit 18, which humidifies the cathode input air. The WVT unit 18 is one type of applicable humidification device; other types of humidification devices may be applicable to humidify the cathode inlet air, such as rotary heat exchangers, evaporators, etc. Cathode exhaust is vented from the stack 12 from a cathode exhaust line 20 through a check valve 22.The exhaust line 20 directs the cathode exhaust gas to the WVT unit 18 to provide the humidity to humidify the cathode input air. A bypass line 28 is provided around the WVT unit 18 to direct some or all of the cathode exhaust gas around the WVT unit 18 to the cathode exhaust gas around the WVT unit 18 in a controlled manner. In an alternative embodiment, the bypass line 28 may be an inlet bypass line. A bypass valve 24 is provided in the bypass line 28 and is controlled to selectively recirculate the cathode exhaust gas through or around the WVT unit to provide a desired amount of humidity to the cathode input air. A nitrogen source 26 is also included to provide nitrogen gas to the cathode side of the stack 12.

[0015] The anode side of the fuel cell stack 12 receives hydrogen gas from a hydrogen source 32 on an anode input side 30 and delivers an anode exhaust gas on line 34 through a valve 36, such as a vent valve, an exhaust valve, etc. A pump 38 pumps a cooling fluid through the stack 12 and through a cooling loop 40 outside the stack 12. A power source 42, such as a battery, is connected to provide a current flow through the stack 12.

[0016] Fig.2 is a flowchart 50 illustrating a process for operating a voltage recovery determination algorithm that determines whether and when to operate a voltage recovery algorithm intended to recover a reversible voltage loss of the fuel cell stack 12 as the stack 12 ages in its lifetime. The algorithm increments an algorithm determination counter while the vehicle is in a key-on state in box 52, which prevents the determination algorithm from running continuously but triggers the determination algorithm to operate at specific time intervals, which are application-specific and could be, for example, every hour of vehicle operation.The counter will increment when the vehicle is in its key-on state, regardless of whether the vehicle is moving or not, and will stop at a time when the vehicle is off, to be incremented again the next time the vehicle is turned on.

[0017] Once the counter reaches the predetermined time interval, the algorithm then proceeds to box 54 to detect an irreversible stack voltage loss V irr In one embodiment, the determination of the estimated value of the irreversible voltage loss V irr on an in-situ measured voltage loss based on determining a remaining catalyst surface area in the fuel cell, as discussed above, although other techniques for determining the irreversible voltage loss V irr can be equally applicable.

[0018] As soon as the algorithm detects the irreversible voltage loss V irr the algorithm then goes to box 56 to calculate a current stack voltage V act One skilled in the art will use different methods to determine the estimated current stack voltage V act of the fuel cell stack 12 at a specific stack current density. One applicable method used in the art is to determine a polarization curve of the fuel cell stack 12, which represents a relationship between the stack current and the stack voltage. US 2008 / 0 182 139 A1 discloses a method for determining a stack voltage using a polarization curve, which is assigned to the applicant of this application.

[0019] Many control parameters for a fuel cell system require a known polarization curve of the fuel cell stack 12, such as knowing the maximum voltage potential and the current draw available from the fuel cell stack 12. As the stack 12 ages, the stack polarization curve also changes as a result of stack degradation. The algorithm of the '139 application estimates two or more stack parameters from collected data of the stack 12 being processed and uses the parameters to calculate the polarization curve using the equation: Ecell=Erev−(j+a)∗RHFR−(0.07∗log10(j+aj0)+c log10(1−jj∞)) where E cell is the cell voltage (V), j is the current density (A / cm 2 ), R HFR the cell HFR resistance (Ohm cm 2 ) is, E rev is the thermodynamically reversible cell potential (V), a is the background current density from a cell short circuit / cell surface current (A / cm 2) is, j 0 the exchange current density (A / cm 2 ) is, j ∞ the limiting current density (A / cm 2 ) and c is the mass transfer coefficient.

[0020] As soon as the determination algorithm detects the irreversible voltage loss V irr and the estimated current stack voltage V atc estimated, a voltage difference ΔV is calculated, which is the difference between the voltages V irr and V act in Box 58. The voltage difference ΔV is a representation of how much voltage the fuel cell stack 12 is capable of producing but does not produce, which should be recoverable by a voltage recovery technique. In other words, the voltage difference ΔV is a representation of the reversible voltage loss of the fuel cell stack 12.

[0021] The determination algorithm then determines whether the voltage difference ΔV is greater than a predetermined threshold voltage in box 60. In one non-limiting embodiment, the determination algorithm determines whether the voltage difference ΔV is greater than a first voltage threshold at 100% of the stack power or greater than a second voltage threshold at 25% of the stack power. The first and second voltage thresholds would be a specific application for a particular vehicle and fuel cell system and may be, for example, 20 mV for the first voltage threshold, with the second voltage threshold typically being slightly lower, although the two values ​​could also be the same. The estimated irreversible voltage loss V irr and the estimated current voltage V actare determined for a possible range of current densities based on the polarization curve estimation models. Therefore, an irreversible voltage loss V irr and an estimated current voltage V act for each stack current density, thus yielding a voltage difference ΔV for each stack current density point. Polarization curve models are well known to those skilled in the art, resulting in the extraction of data points outside the operating range and interpolation between the data points.

[0022] If each of these criteria for a specific stack performance for a voltage difference ΔV is met, then the determination algorithm triggers a voltage recovery algorithm, which is performed in box 62. Those skilled in the art will know different methods for recovering the voltage loss in a fuel cell stack. A non-limiting example can be attributed to US 2011 / 0 195 324 A1. The document describes two techniques, both of which operate the fuel cell stack at a relatively low temperature and have a relative humidity (RH) for the cathode inlet above the cathode exhaust saturation level. The first technique also includes providing hydrogen at the anode side of the stack 12 and air at the cathode side of the stack 12 and operating the stack at a relatively low voltage.The second technique involves flowing hydrogen to the anode side of the stack and nitrogen to the cathode side of the stack 12, as well as from source 26, using an external power source, such as power source 42, to provide a current density and to provide an anode humidity level significantly higher than the cathode humidity level. Other techniques for recovering the stack voltage loss are known to those skilled in the art. Theoretically, the voltage recovery algorithm should recover all or most of the reversible voltages that have been lost, thus reducing the irreversible voltage losses V. irr and the estimated current stack voltage V act are almost the same. However, in practice, the voltage recovery algorithm will have limitations in recovering all the reversible voltage losses.

[0023] Once the voltage recovery has been performed, the algorithm then proceeds to box 64 to reset the counter to zero and continues to box 52 to wait for the next time the counter is greater than a predetermined number.

[0024] If the algorithm determines that the voltage difference ΔV at 100% power level or the efficiency of the power level at 25% of the stack power is not greater than the voltage threshold of decision diamond 60, then the determination algorithm can run another process which determines at decision diamond 66 whether the current stack voltage V act is a predetermined stack voltage at the end of life of the fuel cell stack 12. In particular, in one embodiment, the determination algorithm determines whether the estimated current stack voltage V actat 100% of the stack power is less than the stack voltage at 100% of the power that occurs at the end of life (EoL) of the stack 12, which can be estimated or predicted. The estimated current voltage V act at 100% of the stack power must be at least at minimum level, where if the current stack voltage V act is unable to meet this minimum level, then stack 12 is at the end of its life and must be replaced.

[0025] If the estimated current voltage V actat 100% of the stack power is less than the desired end-of-life voltage in decision diamond 66, then the algorithm adds a forced recovery number to a forced recovery counter in box 68. Each time the forced recovery number is incremented, the algorithm defines whether the forced recovery number is greater than a predetermined number in decision diamond 70 and, if not, the voltage recovery algorithm is executed in box 62, so that a forced voltage recovery process is performed even if the voltage difference ΔV is not greater than the threshold voltage. If the forced recovery number exceeds the predetermined number in decision diamond 70, then the voltage recovery process cannot help enough and the algorithm indicates to the vehicle operator in box 72 that maintenance is required. If the estimated current voltage V actat the maximum stack power is greater than the end-of-life voltage in decision diamond 66, then the algorithm proceeds to box 74 where it sets the forced reclamation counter to zero and then continues to box 64 to also set the counter to zero.

[0026] By considering the voltage difference ΔV as a recovery voltage based on the vehicle's operating conditions, the voltage recovery algorithm is adaptable to different drivers who may drive the vehicle differently. For example, drivers who tend to frequently drive at maximum stack power and cause high voltage transients may require voltage recovery more frequently.

Claims

[1] A method for determining when to perform a recovery process of a stack voltage of a fuel cell stack in a fuel cell system, the method comprising: - Determining an estimate of an irreversible voltage loss of the fuel cell stack; - Determining an estimate of a current voltage of the fuel cell stack; - Determining a difference between the irreversible voltage loss and the current voltage; - Determining whether the difference between the irreversible voltage loss and the current voltage is greater than a predetermined voltage threshold, and which includes determining whether the difference between the irreversible voltage loss and the current voltage at 100% of the stack power is greater than a first predetermined voltage threshold, and determining whether the difference between the irreversible voltage loss and the current voltage at 25% of the stack power is greater than a second predetermined voltage threshold; and - Performing the stack voltage recovery process when one of the differences between the irreversible voltage loss and the actual voltage loss is greater than a predetermined voltage threshold. [2] The method of claim 1, wherein determining an estimate of the irreversible voltage loss, determining an estimate of the current voltage, and determining a difference between the irreversible voltage loss and the current voltage includes providing an estimate of the irreversible voltage loss, determining an estimate of the current voltage, and determining the difference between the irreversible voltage loss and the current voltage over a current density output range of the fuel cell stack. [3] The method of claim 2, wherein determining an estimate of the current voltage of the fuel cell stack includes using a polarization curve that identifies a relationship between the stack voltage and the stack current density. [4] The method of claim 3, wherein the first and second predetermined voltage thresholds are different and the first predetermined voltage threshold is greater than the second predetermined voltage threshold. [5] The method of claim 1, further comprising determining whether the estimated current voltage at 100% stack power is less than a predetermined end-of-life voltage of the stack, and whether the difference between the irreversible voltage loss and the current voltage is not greater than the predetermined voltage threshold. [6] The method of claim 5, further comprising causing the stack voltage recovery process to be performed when the estimated current stack voltage is less than the predetermined end-of-life voltage at 100% of the stack power. [7] The method of claim 6, further comprising: incrementing a forced voltage recovery counter for each time the stack voltage recovery process is performed when the estimated current stack voltage is less than the predetermined end-of-life voltage, and setting a maintenance alert when the forced recovery counter reaches a predetermined number. [8] The method of claim 1, further comprising: periodically incrementing a recovery counter when the system is in a power-up state, and only performing the method for determining when to perform the stack voltage recovery process when the recovery counter reaches a predetermined number.

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