Method for adjusting the platinum oxide fraction in a fuel cell, fuel cell system for a motor vehicle

Abstract

A method (500) for adjusting the platinum oxide fraction in a catalyst layer of a fuel cell (14) of a fuel cell stack (12) for a motor vehicle is described, comprising the following steps: determining (S501) a current fraction of platinum oxide based on a model depending on a current operating state; determining (S502) a voltage or / and a voltage change depending on the current fraction of platinum oxide; adapting (S503) the fraction of platinum oxide depending on an expected or / and a requested operating state of the fuel cell (12, 14) or / and of the vehicle.

Description

Technical Field

[0001] The present invention relates to a method for adjusting the platinum oxide content in the catalyst layer of a fuel cell in a fuel cell stack for motor vehicles, and a fuel cell system for motor vehicles. Background Technology

[0002] When fuel cell systems are used in motor vehicles, platinum oxide forms during operation, particularly during driving, depending on the vehicle's operating conditions and affects the voltage level of the fuel cell stack. To comply with aging standards in the case of the fuel cell stack, it is desirable to maintain a voltage below the critical voltage during operation of the fuel cell system or the vehicle. However, it has been shown that current voltage levels are strongly dependent on parameters related to the platinum oxide content.

[0003] In the case of a fuel cell system, if a high proportion of platinum oxide is present in the fuel cell coupled with high humidity in the membrane, accelerated aging of the fuel cell occurs due to platinum leaching or platinum stripping (or platinum leaching). If the proportion of platinum oxide is low, very high power output can be generated, which, for example, in certain operating types of motor vehicles, such as in stop-and-go city driving, leads to challenges in powertrain management, as the high power of the fuel cell system must be appropriately delivered to the electrical load or stored. This problem is addressed here based on... Figure 6 The following chart provides a brief explanation. This chart schematically shows the voltage-current density characteristics for a high platinum oxide fraction (solid line) and a low platinum oxide fraction (dashed line). Comparing the current density with a constant voltage U reveals that the current density is higher with a low platinum oxide fraction. Correspondingly, this results in a higher minimum power P2 (the product of voltage and current) for the fuel cell at the same (cell) voltage compared to the minimum power P1 with a high platinum oxide fraction. Summary of the Invention

[0004] The purpose of this invention is to describe a method for optimizing the operation of a fuel cell system, thereby preventing premature aging.

[0005] This objective is achieved by a method having the features of claim 1 and by a fuel cell system having the features of claim 11. Advantageous designs with suitable improvements are described in the dependent claims.

[0006] Therefore, a method for adjusting the platinum oxide content in the catalyst layer of a fuel cell stack for motor vehicles is proposed, comprising the following steps: The current share of platinum oxide is determined based on the model according to the current operating status; The voltage and / or voltage variation are determined based on the current fraction of platinum oxide. The amount of platinum oxide is adapted according to the anticipated and / or requested operating conditions of the fuel cell and / or vehicle.

[0007] This method uses a model to continuously calculate the current fraction of platinum oxide and determine its impact on the voltage in the fuel cell. Other factors or parameters can be supplemented by considering this method, such as the partial pressure of the gas supplied to the fuel cell (e.g., hydrogen), the aging state of the fuel cell, and fuel cell contamination.

[0008] In this method, especially during the start-up or shutdown phase of the fuel cell, the proportion of platinum oxide can be reduced by discharging the fuel cell stack to a voltage of less than or equal to 0.5 volts.

[0009] The proportion of platinum oxide can be reduced by adjusting the amount of air supplied to the cathode side, thereby reducing the voltage in the fuel cell or fuel cell stack.

[0010] The proportion of platinum oxide can also be reduced by increasing the output power of the fuel cell or fuel cell stack, thereby lowering the voltage in the fuel cell or fuel cell stack. In this case, the battery can be charged using the power output from one or more fuel cells.

[0011] In this method, the proportion of platinum oxide can be increased or at least maintained by reducing the output power of the fuel cell or fuel cell stack while increasing the power output induced by the cell. This, for example, avoids operating points or conditions where platinum oxide is reduced. In this case, the required power is buffered by the cell or obtained from the cell, thereby optimizing the fuel cell's operating state for the proportion of platinum oxide.

[0012] The platinum oxide fraction can also be increased by generating a negative load jump, wherein a higher power, lower than the reference operating power of the fuel cell, is temporarily or initially adjusted before subsequently adjusting the target power. This load jump achieves a minimum power lower than that in reference operation. In this case, the active platinum oxide control method results in platinum oxide not decomposing specifically at the initially higher power compared to reference operation. Therefore, upon subsequent negative load jumps, a significantly reduced minimum power compared to reference operation can be achieved, thereby increasing the platinum oxide fraction in this scenario.

[0013] With a confirmed high proportion of platinum oxide, the relative humidity of the air supplied to the cathode side can be reduced. This can suppress the degradation or aging of the fuel cell; in particular, the reduced humidity can help prevent platinum detachment or platinum leaching.

[0014] The adaptation to the platinum oxide fraction, specifically decreasing or increasing, can be determined based on a continuously determined voltage gradient and / or a continuously determined efficiency gradient. In this case, the adaptation can be determined based on a time segment (Zeitanteil) during which a specific voltage exists. This allows for time-discrete adjustment, taking into account the variation of the platinum oxide fraction at a specific voltage, or more precisely, according to how long that voltage value exists.

[0015] A fuel cell system for motor vehicles is also proposed, comprising a fuel cell that generates voltage, wherein the fuel cell includes an anode in flow connection to an anode circuit, a cathode in flow connection to a cathode supply section, an ion-conducting membrane separating the cathode from the anode, and a catalyst layer made of platinum oxide. Furthermore, the fuel cell system includes a controller configured to perform the aforementioned method.

[0016] In such a fuel cell system, the cathode supply section may have an air supply line with a humidifier and a humidifier bypass. In this case, at least one valve device may be arranged in the air supply line and / or the humidifier bypass to control the flow through the humidifier bypass. This configuration with a humidifier bypass allows for targeted control or adjustment of the humidity of the airflow supplied to the fuel cell on the cathode side.

[0017] Motor vehicles, especially electric motor vehicles, can be equipped with the aforementioned fuel cell systems. Attached Figure Description

[0018] Other advantages and details of the invention will become apparent from the following description of the embodiments, with reference to the accompanying drawings. Wherein: Figure 1 A fuel cell system with a humidifier bypass is shown; Figure 2 The diagram shows a qualitative representation of the relationship between current and voltage in a fuel cell, as well as the formation and decomposition of platinum oxide. Figure 3 A simplified diagram is shown to illustrate a method for adjusting the platinum oxide content in the catalyst layer of a fuel cell; Figure 4 A graph is shown that qualitatively illustrates the change in power over time under a negative load step; Figure 5 A graph is shown that qualitatively illustrates the relationship between voltage gradient and efficiency gradient, as well as the measures derived therefrom; Figure 6 A graph is shown, which qualitatively and schematically illustrates the relationship between cell voltage and current density at different platinum oxide fractions. Detailed Implementation

[0019] Figure 1 A fuel cell system 10 is schematically and simplified in the diagram. The fuel cell system 10 includes a fuel cell stack 12 having a plurality of fuel cells 14 connected in series. Each of these fuel cells 14 has an anode and a cathode, wherein the anode and cathode are separated from each other by a proton-conducting membrane. The membrane may be formed of an ionomer, such as a sulfonated tetrafluoroethylene polymer (PTFE) or a polymer of perfluorosulfonic acid (PFSA). Alternatively, the membrane may also be implemented as a sulfonated hydrocarbon membrane.

[0020] The catalyst can be incorporated into the anode and / or cathode of the fuel cell 14. In this case, the membranes may have a catalyst layer disposed or coated on their first and / or second sides. Consideration, for example, noble metals such as platinum, palladium, ruthenium, or mixtures thereof as catalysts. In this case, the catalyst acts as a reaction accelerator in the reaction of the corresponding fuel cell 14.

[0021] Anode gas or fuel (e.g., hydrogen) is supplied to the anode via an anode chamber within the fuel cell stack 12. In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The membrane allows protons to pass through but not electrons. In this case, oxidation, or electron release, occurs at the anode. As protons pass through the membrane to the cathode, electrons are conducted to the cathode or electrical load 18 via external circuitry 16. Cathode gas (e.g., oxygen or oxygen-containing air) can be supplied to the cathode via a cathode chamber within the fuel cell stack 12, thereby causing reduction, or electron absorption, to occur on the cathode side. The electrical load 18 can be, for example, a storage device, such as a high-voltage battery in a motor vehicle, or an electric motor, or other device powered by electrical energy, particularly a vehicle component.

[0022] A compressor 20 is arranged on the air side or cathode side, which, for example, draws in and compresses ambient air. Compression increases the temperature of the drawn-in air or cathode gas. Therefore, the cathode gas is first guided via compressor line 22 to a booster air cooler 24 to be cooled again to the desired temperature. From the booster air cooler 24, the drawn-in and compressed cathode gas is directed to a humidifier 26. In the humidifier 26, the dry cathode gas mixes with the moist cathode exhaust gas (which is supplied to the humidifier 26 via cathode exhaust gas line 28) and is thus humidified. The humidified cathode gas is supplied to the cathode chamber of the fuel cell stack 12 via cathode supply line 30. Furthermore, the humidifier 26 is connected to an exhaust line 32, through which the remaining cathode exhaust gas is drawn from the fuel cell system 10.

[0023] A humidifier bypass 29 branches off upstream of the humidifier 28. After the humidifier 28, the humidifier bypass 29 connects back to the cathode supply line 30. At least one valve device 31 may be installed or arranged upstream of the humidifier 28 or in the humidifier bypass 29 to adjust at least a portion of the flow through the humidifier bypass 29. Even if Figure 1 The example shows two valve devices 31, but it is also clear that a single valve device 31 can be sufficient. Therefore Figure 1 The valve device 31 shown can be optionally provided, or can be provided cumulatively. At least one valve device 31 or the only valve device should be arranged such that the mass flow of air can be completely directed through the humidifier 28 or completely through the humidifier bypass. Of course, the valve device 31 can also be arranged such that it can be adjusted to an intermediate position, so that a portion of the mass flow of air is directed through the humidifier 28 and the humidifier bypass 29, respectively.

[0024] In this example, the anode chamber of the fuel cell stack 12 is connected at the stack inlet side to a fuel storage tank 38, which supplies anode gas or fuel, via anode supply lines 34, 36. The fuel cell system 10 has an anode loop 40, in which unreacted fuel or unreacted anode exhaust gas can be resupplyed to the anode chamber at the stack outlet side at the anode via an anode recirculation line 42. For this purpose, the anode recirculation line 42 is connected to the anode supply line 34, such that a portion 36 of the anode supply line 34, together with the anode recirculation line 42, forms the anode loop 40.

[0025] The anode recirculation line 42 has a recirculation fan 44, which is particularly fluid-mechanically connected to the anode recirculation line 42. An ejector pump or injector 46 is arranged in the region at the junction of the anode supply line 34 and the anode recirculation line 42, supporting the circulation of a mixture consisting of anode exhaust gas and fresh anode gas or fuel. To regulate the supply of fresh fuel or anode gas, a fuel regulating valve device 48 is arranged in the anode supply line 34, particularly upstream of the injector 46. This fuel valve device 48 is preferably constructed as a pressure regulating valve. Upstream of the pressure regulating valve 48, a heat exchanger 50, for example in the form of a heat recovery unit, is arranged for regulating the fuel or anode gas.

[0026] A purge-ventilation valve 52 is connected to the anode recirculation line 42 to release the gas mixture located in the anode loop 40 to the environment or to a diluent (not shown). A liquid separator 54, particularly a water separator, may also be arranged in the anode recirculation line 42. A sensor device 58, particularly a pressure / temperature sensor, may be arranged on the reactor inlet side. A sensor device 59, particularly a pressure / temperature sensor, may also be arranged on the reactor outlet side.

[0027] The fuel cell system 10 also includes a controller 60, indicated by dashed lines, which is in communicative connection with different components of the fuel cell system 10. In the current example, the controller 60 is specifically designed to detect the voltage generated by the fuel cell 14 or the fuel cell stack 12. Additionally, the controller 60 can be configured to detect or obtain pressure and / or temperature values ​​at or from sensor devices 58, 59. For visual clarity, the communication connections between the controller 60 and sensor devices 58, 59 are not shown, even if they exist. Furthermore, the communication lines typically present to at least one valve device 31, used to control or regulate the flow of air through the humidifier 28 and / or humidifier bypass 29, are also not shown.

[0028] Figure 2 A simplified view shows the curves of voltage U and current I versus time t for a fuel cell. The graph shows that at high voltages in the range of approximately 0.8 V, platinum oxide (+PtOx) forms in the catalyst layer of the fuel cell. If the voltage drops below 0.8 V, for example, in the range of 0.4 to 0.6 V, platinum oxide decomposes in the catalyst layer (-PtOx). In other words, when the fuel cell power is low (at high voltage), especially during the minimum power operation phase, platinum oxide forms. When the fuel cell power is high (at low voltage), especially during full load operation, platinum oxide decreases or is reduced.

[0029] Figure 2 The formation or decomposition behavior of platinum oxide in the catalyst layer shown can, for example, serve as a basis for process technology adjustments to the platinum oxide share during fuel cell operation.

[0030] Figure 3 The steps of a simplified and schematic method 500 for adjusting the platinum oxide fraction, wherein the platinum oxide fraction is adapted according to the anticipated and / or requested operating conditions of the fuel cell and / or vehicle. In method 500, in step S501, a model-based determination of the current platinum oxide fraction is performed based on the current operating conditions. In step S502, a determination of voltage and / or voltage variations is performed based on the current platinum oxide fraction. Finally, in step S503, the platinum oxide fraction is adapted according to the anticipated and / or requested operating conditions of the fuel cell and / or vehicle.

[0031] In method 500, according to step S504, the proportion of platinum oxide can be reduced by discharging the fuel cell stack to a voltage of less than or equal to 0.5 volts, especially during the start-up or shutdown phase of the fuel cell.

[0032] Alternatively or supplementarily, the proportion of platinum oxide can be reduced by adapting the amount of air supplied on the cathode side, thereby reducing the voltage in the fuel cell or fuel cell stack, as illustrated in step S505.

[0033] In method 500, according to step S506, alternatively or additionally, the proportion of platinum oxide can be reduced by increasing the output power of the fuel cell or fuel cell stack, thereby reducing or decreasing the voltage in the fuel cell or fuel cell stack. In this case, according to step S506, the battery can be charged by means of the power output by one or more fuel cells.

[0034] According to step S507, the platinum oxide fraction can be increased or at least maintained by reducing the output power of the fuel cell or fuel cell stack while increasing the power output induced by the cell. In this case, more energy or power is temporarily obtained from the cell than is provided to the cell by the fuel cell. That is, in this case, the cell acts as an energy buffer or current buffer, where it is desired or intended for the cell to temporarily bear a stronger load to facilitate improved operation of the fuel cell in managing the platinum oxide fraction in the catalyst layer.

[0035] Alternatively or additionally, the platinum oxide fraction can be increased by generating a negative load jump, wherein a higher power, lower than the reference operating power of the fuel cell, is temporarily adjusted before the target power is subsequently adjusted, wherein the minimum power is achieved by the generated load jump, which is lower than that in reference operation, as illustrated in step S508. This type of adjustment of the platinum oxide fraction can be particularly applied during the startup of a fuel cell system.

[0036] exist Figure 4 The simplified graph illustrating the power P versus time t illustrates the method steps applied in step S508. In this graph, the dashed line corresponds to a reference operation of the fuel cell or fuel cell stack. If the described method 500 is used here to adjust the platinum oxide share based on a model, the solid line represents the power trend. It can be seen from this graph that when the fuel cell or fuel cell system starts up, the power initially (especially during the first approximately 20 seconds of operation) remains slightly lower, thereby selectively decomposing less platinum oxide. With subsequent negative load jumps, a significantly reduced minimum power can be reached over a longer period compared to the reference operation. The power curves for operation with the adjusted platinum oxide share (solid line) and the reference operation (dashed line) converge after approximately 150 to 200 seconds. Therefore, optimized operation can be achieved, particularly during the startup phase of the fuel cell or fuel cell system or vehicle, where a favorable state is achieved for the fuel cell in terms of the possible minimum power by taking into account and adjusting the platinum oxide share.

[0037] According to step S509, when a high proportion of platinum oxide is confirmed, the relative humidity of the air supplied to the cathode side can be reduced. In this case, the aforementioned humidifier bypass 29 ( Figure 1 Or at least one associated valve device 31 can be correspondingly controlled by the controller 60 to supply air to the fuel cell on the cathode side while at least partially bypassing the humidifier 28. This achieves the following: less humid air reaches the fuel cell, which inhibits the rinsing of platinum oxide and thus inhibits the deterioration or aging of the fuel cell.

[0038] In method 500, the adaptation of the platinum oxide fraction, particularly reducing or increasing it, can be determined, for example, according to one of the steps S503 to S509 described above, based on a continuously determined voltage gradient and / or a continuously determined efficiency gradient. For this purpose, see [reference needed]. Figure 5 The chart in the diagram simplifies and schematically illustrates which measure to select for adjusting the platinum oxide fraction at which voltage gradient dU.

[0039] Large voltage gradients dU are associated with the targeted startup process of fuel cells, particularly with negative load transitions, as described in the above-mentioned reference method steps S508 and... Figure 4 As described. For example, this could be a targeted discharge of the fuel cell stack at an appropriate time, where, for example, the cell voltage is reduced by 30mV, thereby reducing the fuel cell efficiency by, for example, 2%.

[0040] At a moderate voltage gradient, for example, the power of the fuel cell can be increased and output to the battery, as explained with reference to step S506. A shift in the load point, for example, would cause a 20mV decrease in battery voltage, whereby the fuel cell efficiency would decrease by, for example, by 4%.

[0041] At low voltage gradients dU, the inflow to the cathode side is affected, and in particular, the humidity of the air is also affected, as described with reference to step S509. By changing, and in particular reducing, the amount of air supplied, for example, the battery voltage can be reduced by 10 mV, wherein the fuel cell efficiency is reduced by, for example, 1%.

[0042] In method 500, particularly when considering voltage gradients, the fit can be determined based on time segments during which a specific voltage exists. This ensures that the development of the platinum oxide share is also considered during specific, and especially longer, time periods while maintaining the voltage.

Claims

1. A method (500) for adjusting the platinum oxide content in a catalyst layer of a fuel cell (14) in a fuel cell stack (12) for a motor vehicle, comprising the steps of: The current share of platinum oxide (S501) is determined based on the model according to the current operating status; The voltage and / or voltage change is determined (S502) based on the current share of platinum oxide; The proportion of platinum oxide is adapted (S503) to the expected and / or requested operating conditions of the fuel cell (14) and / or the motor vehicle. The characteristic feature is that the proportion of platinum oxide (S507) is increased or at least maintained by reducing the output power of the fuel cell or the fuel cell stack while increasing the power output induced by the cell. Specifically, the appropriate platinum oxide fraction is determined based on a continuously determined voltage gradient and / or a continuously determined efficiency gradient.

2. The method (500) according to claim 1, wherein, During the start-up or shutdown phase of the fuel cell, the proportion of platinum oxide is reduced (S504) by discharging the fuel cell stack to a voltage of less than or equal to 0.5 volts.

3. The method (500) according to claim 1 or 2, wherein, The voltage in the fuel cell or fuel cell stack is reduced by adjusting the amount of air supplied to the cathode side (S505) to reduce the proportion of platinum oxide.

4. The method (500) according to any one of claims 1 to 3, wherein, The voltage in the fuel cell or fuel cell stack is reduced by increasing the output power of the fuel cell or fuel cell stack to reduce the proportion of platinum oxide (S506).

5. The method (500) according to claim 4, wherein, The battery (18) is charged by the power output from one or more of the fuel cells (14).

6. The method (500) according to any one of the preceding claims, wherein, The proportion of platinum oxide (S508) is increased by generating a negative load jump, wherein a higher power is temporarily adjusted first before the target power is subsequently adjusted, the higher power being lower than the reference operating power of the fuel cell, wherein the minimum power lower than that in the reference operation is achieved by the generated load jump.

7. The method (500) according to any one of the preceding claims, wherein, In the case of a high proportion of platinum oxide, the relative humidity of the air supplied to the cathode side is reduced (S509).

8. The method (500) according to claim 1, wherein, The adaptation is determined based on the time segments during which a specific voltage exists.

9. A fuel cell system (10) for a motor vehicle, comprising: A fuel cell (14) that generates voltage, wherein, The fuel cell (14) includes an anode that is in flow connection with an anode circuit (40), a cathode that is in flow connection with a cathode supply section, an ion-conducting membrane that separates the cathode from the anode, and a catalyst layer made of platinum oxide. A controller (60) configured to perform the method according to any one of the preceding claims.

10. The fuel cell system (10) according to claim 9, wherein, The cathode supply unit has an air supply line (30) with a humidifier (28) and a humidifier bypass (29) that bypasses the humidifier (28).

11. The fuel cell system (10) according to claim 10, wherein, At least one valve device (31) is arranged in the air supply line (30) and / or the humidifier bypass (29) to control the flow through the humidifier bypass (29).

12. A motor vehicle having a fuel cell system according to any one of claims 9 to 11.

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