Fuel cell operation method for regeneration of cathode catalyst

Abstract

The invention relates to a method for operating a proton exchange membrane fuel cell system having at least one fuel cell stack for regenerating a cathode catalyst of the fuel cell system as required, the method has the following steps: supplying hydrogen and oxygen to the fuel cell system for performing a fuel cell process in a normal operating phase; continuously and / or repeatedly detecting at least one operating parameter for evaluating the performance of the fuel cell system; starting a temporary regeneration phase of the at least one fuel cell stack, comprising providing external electrical power for compensating for the electrical power of the associated fuel cell stack, interrupting the supply of oxygen to the associated fuel cell stack, introducing a purge gas into the cathode section of the associated fuel cell stack, and after a predetermined purge duration has elapsed, starting the at least one fuel cell stack. And stopping the temporary regeneration phase for continuing the normal operation phase.

Description

Technical Field

[0001] The invention relates to a method for operating a proton exchange membrane (PEM) fuel cell system having at least one fuel cell stack for regenerating a cathode catalyst of the fuel cell system as required, and also to a fuel cell system. Background Art

[0002] Hydrogen-based fuel cells require hydrogen and oxygen or a gas mixture with oxygen to carry out the fuel cell process. The waste heat generated by the fuel cell is usually discharged by means of a cooling circuit and discharged to the surrounding environment via the main vehicle radiator when installed in a motor vehicle. For applications with high power requirements, such as buses and trucks, multiple fuel cell systems are usually used in parallel with each other. Long-term uninterrupted operation of the fuel cell can lead to power losses, which can be demonstrated by measuring different parameters of the individual fuel cells. The power loss is often reversible and can often be reversed by simple system measures. For example, platinum nanoparticles can be oxidized on the cathode catalyst, so that the reduction surface is reduced. This situation can be reversed by so-called bleeding of the cathode gas, that is, shutting off the oxygen supply in the operating fuel cell.

[0003] However, it is conceivable that the oxidized platinum penetrates deeply into the cathode catalyst. If this is the case, the oxygen starvation may not be sufficient for regeneration and the degradation of the associated fuel cell may be permanent. Summary of the invention

[0004] It is therefore an object of the present invention to provide a method in which the cathode catalyst can be regenerated as required even if oxidized platinum has already penetrated deeply into the cathode catalyst.

[0005] This object is achieved by a method for operating a PEM fuel cell system having the features of independent claim 1. Advantageous embodiments and developments are apparent from the dependent claims and the following description.

[0006] A method for operating a PEM fuel cell system is proposed, which has at least one fuel cell stack for regenerating the cathode catalyst of the fuel cell system as needed, the method comprising the following steps: supplying hydrogen and oxygen to the fuel cell system for performing the fuel cell process in a normal operating phase; continuously and / or repeatedly detecting at least one operating parameter for evaluating the performance of the fuel cell system and starting a temporary regeneration phase of at least one fuel cell stack, which includes providing external electrical power for compensating the electrical power of the relevant fuel cell stack; interrupting the supply of oxygen to the relevant fuel cell stack; introducing a purge gas into the cathode section of the relevant fuel cell stack, and stopping the temporary regeneration phase after a predetermined flushing time to continue the normal operating phase.

[0007] The fuel cell system preferably has a plurality of fuel cells which are combined into one or more fuel cell stacks. When used in trucks, it is particularly advantageous to use proton exchange membrane (PEM) fuel cells. The fuel cell is supplied with hydrogen or a gas containing hydrogen on the anode side and preferably with air on the cathode side. As mentioned at the outset, a plurality of fuel cell stacks can be used flexibly in parallel with one another to provide different powers.

[0008] The fuel cell system is initially operated largely or temporarily stably by supplying hydrogen and oxygen. This corresponds to the normal operating phase. Therefore, the fuel cell system is supplied with adjustable volume flows of air and hydrogen so that the fuel cell process can proceed. To maintain this process, current is required, for example through corresponding electrical consumers.

[0009] During normal operation, operating parameters, such as individual cell voltages, can be detected in order to enable an assessment of performance. If, for example, the individual cell voltage drops by a certain amount or a certain percentage, such as 5% or more, from the nominal cell voltage, the regeneration phase can be started. The detection can be performed in a time-controlled manner, such as a certain time after the relevant fuel cell stack is interrupted from operation, such as 30 to 40 minutes. Alternatively, as mentioned above, the current voltage on the relevant fuel cell stack can be compared with a reference value when performing BOL (begin of life) or when performing BOD (begin of drive). When there is a large performance deviation, such as 20%, the regeneration according to the present invention is turned on. Another alternative is to compare the measured cell voltage, especially the worst value, with a reference value. In addition, alternatively, the impedance value of the relevant fuel cell stack or battery cell can be detected. Alternatively, when, for example, other methods purely involving discharge cannot lead to complete regeneration, the regeneration according to the present invention can be turned on. This can be determined, for example, based on a voltage deviation of at least 5% from the expected value.

[0010] In the regeneration phase, the reduced power of the relevant fuel cell stack is first compensated so that the operation of the consumers of the fuel cell system can be maintained. This may include, for example, the operation of additional fuel cell stacks, the release of electrical power from batteries, etc.

[0011] The air or oxygen supply is interrupted from normal operation, wherein the hydrogen supply can be maintained at least briefly during this process. In addition to the diffusion of hydrogen through the membrane to the cathode side, an increased oxygen consumption is forced in the form of a purge gas supply to the cathode side, and thus a strong drop in the cathode potential or cell potential. The reducing conditions at the cathode serve to reduce oxide deposition on the catalyst. This process is also referred to as "hydrogen soaking (H2S)". 2 Soak).

[0012] By at least partially maintaining the current flow, reducing conditions can be rapidly adjusted in the relevant fuel cell stack. The lack of air enhances the cleaning of the cathode catalyst by causing a voltage drop across the relevant fuel cell and accompanying reduction of the platinum oxide, which results in an improvement in the active catalyst surface.

[0013] If such a regeneration phase ends, for example after a short flushing time of 1 s, 2 s, 3 s or generally after a few seconds, normal operation can be continued. It is conceivable to perform such a regeneration phase regularly in order to maintain the performance of the fuel cell system.

[0014] In an advantageous embodiment, the introduction of the purge gas into the cathode section comprises supplying the purge gas into the cathode outlet. The effort for integrating this function is minimal, since the usual connection between the purge line and the exhaust line downstream of the cathode shutoff valve can be modified by connecting the purge line to the side of the cathode shutoff valve that faces upstream.

[0015] In an advantageous embodiment, interrupting the supply of oxygen comprises opening a fuel cell bypass and closing a cathode shutoff valve, wherein the cathode shutoff valve is arranged downstream of the cathode outlet and wherein the fuel cell bypass is connected to the cathode shutoff valve downstream of the cathode shutoff valve.

[0016] In an advantageous embodiment, the method further comprises closing the cathode inlet valve. This directly results in an oxygen depletion which is additionally accelerated by the introduction of the purge gas.

[0017] In one advantageous embodiment, the fuel cell system has a plurality of fuel cell stacks, wherein the introduction of the purge gas comprises supplying the purge gas of a first fuel cell stack into the cathode section of a second fuel cell stack. The duration and intensity of flushing with the purge gas can thereby be controlled particularly easily, since the purge gas is obtained from an independent source.

[0018] In an advantageous embodiment, after stopping the temporary regeneration phase and after a subsequent predetermined waiting period, the normal operating phase is continued.

[0019] The present invention also relates to a fuel cell system, which has at least one fuel cell stack, the fuel cell stack having an anode and a cathode; a purge gas line connected to the anode outlet, the purge gas line having a valve arranged thereon; and a control unit, wherein the purge gas line is capable of being connected to the cathode outlet of at least one fuel cell stack, and wherein the control unit is coupled to the at least one fuel cell stack and the valve arranged on the purge gas line and is constructed for performing the method according to the above description.

[0020] In an advantageous embodiment, the purge line is connected to a purge valve at the anode outlet of the fuel cell stack and to a cathode outlet of the same fuel cell stack.

[0021] In an advantageous embodiment, the purge line is connected to the anode outlet of a fuel cell stack and to a purge diverter valve, wherein the purge diverter valve is connected to the cathode outlet of another fuel cell stack.

[0022] In an advantageous embodiment, the fuel cell system further comprises a purge valve for each fuel cell stack, wherein the purge valves are connected to the exhaust gas line downstream of the cathode shut-off valve. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following describes in detail the measures to improve the present invention together with the description of the preferred embodiments of the present invention according to the accompanying drawings.

[0024] The accompanying drawings show:

[0025] Figure 1 : Schematic diagram of the fuel cell system;

[0026] Figure 2 : Block diagram of a method for operating a fuel cell system;

[0027] Figure 3 : Schematic diagram of another fuel cell system;

[0028] Figure 4 : Block diagram of another method for operating a fuel cell system. DETAILED DESCRIPTION

[0029] Figure 1 The fuel cell system 2 is schematically shown in a block diagram. The fuel cell system 2 has a control unit 3 which is coupled to the functional components of the fuel cell system 2 .

[0030] Furthermore, only a single fuel cell stack 4 is shown by way of example, which comprises an anode 6 or anode segment 6, a cathode 8 or cathode segment 8 and a membrane (not shown in detail here) and a heat exchanger 10 for dissipating heat. The anode segment 6 and the cathode segment 8 are connected here by way of example only to a DC transformer 12, which converts the voltage provided by the fuel cell stack 4 into the desired level.

[0031] The anode 6 is supplied with hydrogen from a hydrogen tank 14 to which is connected a hydrogen shutoff valve 16, a hydrogen heat exchanger 18, a hydrogen pressure regulator 20 and an exemplary jet pump 22. The jet pump 22 is connected to a compressor 24, and the anode residual gas from the anode outlet 26 is compressed and introduced back to the anode inlet 28.

[0032] A line 29, a purge valve 30, a water separator 32 and a water container 34 are connected to the anode outlet 26. The water container is connected to a discharge valve 36, which can be opened as required to discharge water. The line 29 is used to guide the anode residual gas and to discharge the purge gas. The line 29 is therefore also referred to as a purge gas line within the scope of the present invention.

[0033] The cathode 8 is supplied with air 38, which is filtered via an air filter 40 and compressed via an air compressor 42. An air heat exchanger 44 is connected here, which is connected upstream of the cathode inlet valve 46. Thus, compressed and cooled air flows into the cathode inlet 48 and oxygen-enriched air flows out of the cathode outlet 50. A cathode shut-off valve 52 is connected to the cathode outlet, and a pressure regulator 54 is subsequently connected downstream, via which the exhaust gas reaches an exhaust line 55 and ultimately reaches the surroundings. The air from the air compressor 42 can be directly supplied to the pressure regulator 54 via a fuel cell stack bypass 56 and discharged into the surroundings.

[0034] The purge valve 30 is connected to the cathode outlet 50 in order to flush the cathode section with hydrogen as required, controlled by the control unit 3, in order to perform regeneration of one or more cathode catalysts. Figure 2 The method shown in .

[0035] Only for the sake of completeness, the vehicle radiator 58 is mentioned, which is coupled to the fuel cell heat exchanger 10 and circulates a coolant through the fuel cell heat exchanger 10 and the vehicle radiator 58 by means of a coolant pump 60 .

[0036] Figure 2A method 62 is shown that can be executed by the control unit 3 for operating the fuel cell system 2 to regenerate the cathode catalyst as required. The method 62 first carries out the step of supplying hydrogen and oxygen to the fuel cell system 2 for performing the fuel cell process in the normal operation phase, which is not shown in detail here. At least one operating parameter, such as the cell voltage, can be detected 64 continuously and / or repeatedly to evaluate the performance of the fuel cell system 2. If a limited performance is identified, for example due to a deviation of the detected cell voltage from the expected cell voltage that is outside the tolerance, a temporary regeneration phase is started. The regeneration phase includes providing 66 external electrical power for compensating the electrical power of the relevant fuel cell stack 4, interrupting 68 the supply of oxygen to the relevant fuel cell stack 4 by opening the fuel cell bypass 56 and the pressure regulator 54, so that the air from the air compressor 42 flows almost only past the fuel cell stack 4. For example, the cathode shut-off valve 52 is closed 70. The purge valve 30 and / or the discharge valve 36 are opened so that the purge gas is introduced 72 into the cathode outlet 50. Optionally, the cathode inlet valve 52 can be closed 74. After the regeneration phase, the purge valve 30 and / or the outlet valve 36 are opened again 76. Optionally, a predetermined waiting time can be waited for 78 to elapse. The supply 66 of external electrical power is interrupted 80, the cathode valve 46 and / or 52 is opened again 82 and the normal operation of the fuel cell system 2 is continued 84.

[0037] Figure 3 The schematic diagram shows a fuel cell system 86 having two fuel cell stacks 88 and 90 together with a control unit 3. Figure 1 , the purge gas from the fuel cell stack 90 is directed to the cathode outlet 50 of the fuel cell stack 88 in order to perform regeneration there. Here, the purge valves 30 belonging to the two fuel cell stacks are arranged downstream of the cathode shut-off valve 52, as is common in the prior art. Instead, a purge gas line 91 leading to an additional purge transfer valve 92 is provided, which transfers the purge gas of one fuel cell stack 90 to the other fuel cell stack 88. The purge transfer valve 92 is connected to the anode outlet 26 of one fuel cell stack 90 and the cathode outlet 50 of the other fuel cell stack 88.

[0038] The fuel cell stack bypass 56 of the flushed fuel cell stack 88 is open, as is its pressure regulator 54. The air compressor 42 is activated.

[0039] The fuel cell stack 90 venting the purge gas may be operated such that the hydrogen shutoff valve 16 is open and the hydrogen pressure regulator 20 is open. However, the purge valve 30 is closed and the exhaust valve 36 is closed. The compressor 24 is in operation.

[0040] Figure 4The final modified method 94 is shown, in which the purge diverter valve 92 is opened 96 instead of opening 72 the purge valve and / or the drain valve. Similarly, the purge diverter valve 92 is subsequently closed 98 .

Claims

1. A method (62, 94) for operating a proton exchange membrane fuel cell system (2, 86), the proton exchange membrane fuel cell system having at least one fuel cell stack (4, 88, 90), for regenerating a cathode catalyst of the fuel cell system (2, 86) as required, the method comprising the following steps: supplying hydrogen and oxygen to the fuel cell system (2, 86) for carrying out the fuel cell process in a normal operating phase, continuously and / or repeatedly detecting (64) at least one operating parameter for evaluating the performance of the fuel cell system (2, 86), Initiating a temporary regeneration phase of the at least one fuel cell stack (4, 88, 90), comprising: - providing (66) external electrical power for compensating the electrical power of the associated fuel cell stack (4, 88, 90), - interrupting (68) the supply of oxygen to the relevant fuel cell stack (4, 88, 90), - introducing (72) a purge gas into the cathode section (8) of the relevant fuel cell stack (4, 88, 90), - After a predetermined flushing time has elapsed, the temporary regeneration phase is stopped in order to continue (84) the normal operating phase.

2. The method (62, 94) according to claim 1, in, Introducing (72) a purge gas into the cathode section (8) includes supplying the purge gas into a cathode outlet (50).

3. The method (62, 94) according to claim 2, in, interrupting (68) the supply of oxygen includes opening the fuel cell bypass (56) and closing the cathode shutoff valve (52), The cathode cut-off valve (52) is arranged downstream of the cathode outlet (50), and The fuel cell bypass (56) is connected to the cathode stop valve (52) downstream of the cathode stop valve.

4. The method (62, 94) of any preceding claim, further comprising closing (74) the cathode inlet valve (46).

5. The method (62, 94) according to any one of the preceding claims, in, The fuel cell system (2, 86) has a plurality of fuel cell stacks (4, 88, 90), and Wherein, introducing (72) the purge gas includes supplying the purge gas of the first fuel cell stack (4, 88, 90) to the cathode section of the second fuel cell stack (4, 88, 90).

6. The method (62, 94) according to any one of the preceding claims, in, After stopping the temporary regeneration phase and after a subsequent predetermined waiting period has elapsed (78), the normal operating phase is continued (84).

7. A fuel cell system (2, 86), comprising: at least one fuel cell stack (4, 88, 90) having an anode segment (6) and a cathode segment (8), a purge gas line (29, 91) connected to the anode outlet (26), the purge gas line having a valve (30, 36, 92) arranged thereon, and Control unit (3), in, The purge gas pipeline (29, 91) is connectable to the cathode outlet (50) of the at least one fuel cell stack (4, 88, 90), and The control unit (3) is coupled to the at least one fuel cell stack (4, 88, 90) and to a valve (30, 36, 92) arranged on the purge gas line (29, 91) and is designed to carry out the method (62, 94) according to any of the preceding claims.

8. The fuel cell system (2, 86) according to claim 7, in, The purge gas line (29, 91) is connected to a purge valve (30) at an anode outlet (26) of the fuel cell stack (4, 88, 90) and to a cathode outlet (50) of the same fuel cell stack (4, 88, 90).

9. The fuel cell system (2, 86) according to claim 7, in, The purge gas pipeline (29, 91) is connected to the anode outlet (26) of the fuel cell stack (4, 88, 90) and the purge transfer valve (92), and The purge transfer valve (92) is connected to the cathode outlet (50) of another fuel cell stack (4, 88, 90).

10. The fuel cell system (2, 86) according to claim 9 further comprises a purge valve (30) for each fuel cell stack (4, 88, 90), wherein: Each purge valve (30) is connected to an exhaust pipeline (55) downstream of the cathode stop valve (52).

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