Method for operating a fuel cell system, fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1) with at least one fuel cell stack (100) having a cathode (110) and an anode (120), in which, during standard operation of the fuel cell system (1), the cathode (110) is supplied with air via an air supply (111) and exhaust air leaving the fuel cell stack (100) is discharged via an air outlet (112) and hydrogen is supplied to the anode (120) via an anode circuit (121). According to the invention, exhaust air is introduced into the anode circuit (121) of the anode (120) from the air outlet (112) or, occasionally, from the air outlet (212) of the other fuel cell stack (200) in order to build up a passivation layer of the anode (120) and / or to repassivate the passivation layer of the anode (120). The invention further relates to a fuel cell system (1) for carrying out the method according to the invention.

Description

[Technical field]

[0001] The present invention relates to a method for operating a fuel cell system with the features of the preamble of claim 1. Furthermore, the present invention relates to a fuel cell system suitable for performing said method or operable according to said method.

[0002] An advantageous field of application is fuel cell vehicles, preferably fuel cell vehicles with start-stop operation. [Background technology]

[0003] A fuel cell is an electrochemical energy converter. Hydrogen (H2) and oxygen (O2) can be used inter alia as reactant gases. These are converted by the fuel cell into electrical energy, water (H2O) and heat. At the heart of the fuel cell is the membrane-electrode-arrangement (MEA), which contains a membrane that is already coated with catalytic material to form the electrodes. During operation of the fuel cell, hydrogen is supplied to one electrode, the anode, and oxygen is supplied to the other electrode, the cathode.

[0004] In practice, many fuel cells are combined to form a fuel cell stack or stack to increase electrical output, and multiple fuel cell stacks or systems can be interconnected.

[0005] During standard operation of a fuel cell system, no oxygen is supplied to the anode of the fuel cell stack. When the fuel cell stack is driven down or shut down, the same measures are taken to prevent the degradation of the fuel cell performance. However, the lack of oxygen in the anode leads to the peeling off of the passivation layer on the anode surface, which in turn results in a higher incidence of corrosion. Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention is to reduce the incidence of corrosion on the anode surface by targeted passivation or repassivation, thereby increasing the service life of the anode components. [Means for solving the problem]

[0007] To achieve this object, a method is proposed with the features of claim 1. Advantageous embodiments of the invention can be read from the dependent claims. Furthermore, a fuel cell system suitable for carrying out the method or operable according to the method is described.

[0008] Proposed is a method of operating a fuel cell system comprising at least one fuel cell stack having a cathode and an anode. During standard operation of the fuel cell system, the cathode is supplied with air via an air supply and exhaust air leaving the fuel cell stack is exhausted via an air exhaust. The anode is supplied with hydrogen via an anode circuit. According to the invention, exhaust air is introduced into the anode circuit of the anode from the air exhaust or, occasionally, diverted from the air exhaust of the other fuel cell stack, in order to build up and / or repassivate the passivation layer of the anode.

[0009] Since the exhaust gas of a fuel cell stack usually has a very low oxygen concentration, it can be introduced into the anode circuit and used for passivation or repassivation of the anode surface. Thus, decomposition of the passivation layer during shutdown phases can be easily prevented. If the fuel cell system contains only one fuel cell stack, only one air outlet needs to be used, from which the exhaust air required for passivation or repassivation is diverted. If the fuel cell system contains several fuel cell stacks, the exhaust air of the other fuel cell stacks can be used for passivation or repassivation.

[0010] The creation or attainment of a passivation layer reduces the risk of corrosion and therefore improves the lifespan of the fuel cell stack, and in particular of the electrochemical membrane made of catalytic material, since, among other things, migration of metal ions into the membrane is limited.

[0011] If the required exhaust air is branched off from the air exhaust of the same fuel cell stack, this is advantageously introduced via a purge valve and / or a drain valve that is integrated into the anode circuit and is connected to the air exhaust of the same fuel cell stack via a connecting pipe. Since the purge valve and / or the drain valve are provided according to the standard and are also connected to the air exhaust via a connecting pipe according to the standard, it is possible to utilize the components already provided for carrying out the method in this respect. In this case, the method is completely system-neutral since no additional components are required. Since the connecting pipe is usually used to dilute the cleaning agglomerates discharged via the purge valve and / or the drain pipe by directing them into the air exhaust, it is only necessary to temporarily reverse the flow direction in the connecting pipe.

[0012] For the reversal of the flow direction, it is proposed to temporarily increase the pressure in the air outlet compared to the pressure in the anode circuit, for example by 20 mbar, so that due to the pressure difference, the exhaust air flows from the air outlet into the anode circuit when the purge and / or drain valves are opened.

[0013] In so far as the required exhaust air is branched off from the air exhaust of the other fuel cell stack, it is introduced into the anode circuit of the first fuel cell stack via a separate connecting pipe with an integrated shut-off valve, i.e. at least one additional connecting pipe and an additional valve are provided to connect the anode circuit of the first fuel cell stack with the air exhaust of the other fuel cell stack. The advantage of said separate connecting pipe is that it is not necessary to increase the pressure in the air exhaust compared to the pressure in the anode circuit of the same fuel cell stack for a short time. It has been found that the use of the exhaust air of the external fuel cell stack is preferable compared to the use of the intrinsic exhaust air, since with the increase in pressure the pressure difference in the cell diaphragm may also change, which may lead in the long term to fatigue of the diaphragm material and its failure.

[0014] If the fuel cell system contains several fuel cell stacks, each anode circuit of one fuel cell stack can be connected to the air exhaust of the other fuel cell stacks via a separate connecting pipe with an integrated shut-off valve. In this way, the anode surfaces of all fuel cell stacks can be passivated or repassivated from time to time with the exhaust air of the respective other fuel cell stacks. The number of additional connecting pipes and valves then advantageously corresponds to the number of fuel cell stacks.

[0015] Furthermore, in the case of a fuel cell system with several fuel cell stacks, it is proposed to temporarily increase the total pressure level of the other fuel cell stacks with respect to that of the first fuel cell stack. Unlike the above-mentioned increase in the pressure in the air outlet compared to the pressure in the anode circuit of the same fuel cell stack, the pressure on the cathode side and the pressure on the anode side remain interconnected at all times, so that the pressure difference in the cell diaphragm cannot cause damage. By temporarily increasing the total pressure level of the other fuel cell stack, it is ensured that the exhaust air can flow from its air outlet into the anode circuit of the connected fuel cell stack by opening a shut-off valve integrated in the connecting pipe.

[0016] It is furthermore advantageous to temporarily reduce the oxygen concentration of the exhaust air in the air outlet for passivation or repassivation, which is effective in each case for the air outlet into which the exhaust air required for passivation or repassivation branches off, more precisely regardless of whether it is an air outlet of the same fuel cell stack or of another fuel cell stack. By reducing the oxygen concentration, the effectiveness of the method can be further improved.

[0017] Reducing the oxygen concentration can be accomplished in a variety of ways, such as by limiting air superstoichiometry, by increasing the flow without adjusting the air supply, and / or by increasing the exhaust air feedback rate.

[0018] As a further construction measure, it is proposed to operate a blower integrated in the anode circuit during the introduction of the exhaust air into the anode circuit, which can be used to intensify the circulation of the exhaust air introduced into the anode circuit, so that it is better distributed throughout the anode circuit, in particular a recirculation blower, which is used to recirculate the anode gas leaving the fuel cell stack in the anode circuit during standard operation of the system.

[0019] Furthermore, in order to achieve the object mentioned at the beginning, a fuel cell system is proposed with a number of fuel cell stacks. The fuel cell stacks of the fuel cell system each have a cathode and an anode, where the cathodes are each connected to an air supply on the supply side and to an air outlet on the outlet side. The anodes are each connected to an anode circuit. According to the invention, the air outlet of at least one fuel cell stack can be connected to the anode circuit of the other fuel cell stacks via a separate connecting pipe with an integrated shut-off valve.

[0020] The proposed fuel cell system is therefore suitable for carrying out or can be operated according to the method, so as to obtain the same advantages. In particular, the anode surface of at least one fuel cell stack can be passivated or repassivated using the exhaust air of the other fuel cell stack. As a result, the life span of the fuel cell stack is thus improved.

[0021] Ideally, each fuel cell stack in a fuel cell system has an anode circuit that is connected to the air exhaust of the other fuel cell stacks via a separate connecting pipe with an integrated shutoff valve, so that the exhaust air of the other fuel cell stacks can be used to perform passivation or repassivation of the anode surface, correspondingly improving the life span of all fuel cell stacks.

[0022] Advantageous embodiments of the invention will now be explained in more detail with the aid of the accompanying drawings, in which: FIG. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system. [Diagram 2] FIG. 2 shows a timeline of a method according to the invention that can operate the fuel cell system of FIG. 1. [Diagram 3] FIG. 2 is a schematic diagram of another fuel cell system according to the present invention. [Figure 4] FIG. 4 shows a timeline of a method according to the invention that can operate the fuel cell system of FIG. 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 shows a first fuel cell system 1 suitable for carrying out the method according to the invention or operable according to such a method. The fuel cell system 1 of FIG. 1 comprises a fuel cell stack 100 with a cathode 110 and an anode 120. The cathode 110 is supplied with air as oxygen supply via an air supply 111. The air is taken from the surroundings and firstly fed to an air filter 114. The air is then compressed using an air conveying and compression system 113. As the air warms up during compression, it is cooled using a heat exchanger 115 integrated in the air supply 111 and, if necessary, humidified using a humidifier 116 also integrated in the air supply 111. However, the heat exchanger 115 and / or the humidifier 116 are not necessarily required. On the discharge side, the fuel cell stack 100 is coupled to an air discharge 112 which passes through the humidifier 116, so that the humidified discharge air is available for humidifying the supply air. Downstream of the humidifier 116, the exhaust air is fed to a turbine 131 of the air conveying and compression system 113, by means of which part of the energy previously used for compression can be recovered. The exhaust air is discharged from the air discharge channel 112 via a pressure regulating valve 130 arranged downstream of the turbine 131. To bypass the fuel cell stack 100, a bypass channel 118 and a bypass valve 119 are provided. Via the bypass channel 118 and the bypass valve 119, the air supply channel 111 can be connected to the air discharge channel 112. To prevent a reverse air flow, a check valve 117 is integrated in the air supply channel 111 and in the air discharge channel 112, respectively.

[0025] The anode 120 of the fuel cell stack 100 is supplied with anode gas via an anode circuit 121. In this case, hydrogen may be of particular concern. As the anode gas leaving the fuel cell stack 100 usually still contains hydrogen, it is recirculated via the anode circuit 121, more precisely passively by means of a jet pump 124 and actively by means of a blower 123. Nitrogen accumulates over time in the recirculated anode gas, so that the anode circuit 121 is cleaned from time to time. For this purpose, a purge valve 122 is integrated in the anode circuit 121, which is connected via a connecting pipe 132 to the air discharge 112, so that the cleaned agglomerates can be introduced into the air discharge 112. In the air discharge 112, a rarefaction is achieved which prevents the cleaned agglomerates, which may still contain hydrogen, from mixing with the discharge air, resulting in an explosive gas mixture. The water produced during operation of the fuel cell stack 100 can be separated by means of a water separator 126 integrated in the anode circuit 121 and collected in a container 127. When necessary, the container 127 can be emptied by opening a drain valve 128. Emptying is carried out in a connecting pipe 132, since anode gases can escape together with the water. Furthermore, the heat produced during operation is removed by means of a cooling circuit 129.

[0026] The fuel cell system 1 depicted in Figure 1 may be operated according to the method described below and depicted in Figure 2. The method is used to build up the anode side passivation layer or repassivate such.

[0027] In step S10, the passivation of the anode surface of the fuel cell stack 100 is started. In the next step S11, the pressure in the air discharge line 112, more precisely the pressure upstream of the turbine 131, is slightly increased compared to the pressure in the anode circuit 121, so that a pressure difference of, for example, 20 mbar is achieved. In an optional step S12, the oxygen concentration of the exhaust air in the air discharge line 112 is then reduced. In step S13, the purge valve 122 and / or the drain valve 128 are then opened. Due to the pressure difference between the pressure in the air discharge line 112 and the pressure in the anode circuit 121, the exhaust air then flows from the air discharge line 112 through the connecting pipe 132 into the anode circuit 121 in the opposite direction to the flow direction (see arrow in FIG. 1). In step S14, it is checked whether a certain amount of passivation and / or passivation time, for example 2 seconds, has been achieved. If the result of the check is positive ("yes"), the purge valve 122 and / or the drain valve 128 can be closed again in step S15. Furthermore, as long as step S12 has been performed, the oxygen concentration in the air discharge passage 112 can be increased to a standard level again in step S16. In addition, in step S17, the preset pressure difference between the pressure in the air discharge passage 112 and the pressure in the anode circuit 121 is eliminated. Then, the method ends in step S18.

[0028] A further aspect of the present invention can be achieved using a fuel cell system 1 including a plurality of fuel cell stacks 100, 200. Such a fuel cell system 1 is illustrated in FIG.

[0029] FIG. 3 shows a fuel cell system 1 according to the invention with a first fuel cell stack 100 and a second fuel cell stack 200. The fuel cell stacks 100, 200 each have a cathode 110, 210 and an anode 120, 220. The cathodes 110, 210 are supplied with air as oxygen supply via air supply lines 111, 211, respectively. Air is taken from the surroundings and fed to an air conveying and compression system 113, 213 via air filters 114, 214, providing a certain air mass flow and a certain pressure level. The air heats up in the process, so it is cooled by means of heat exchangers 115, 215 integrated in the air supply lines 111, 211 and humidified by means of humidifiers 116, 216. The exhaust air of the fuel cell stacks 100, 200 is exhausted via air exhaust lines 112, 212, respectively. A turbine 131, 231 for energy recovery and a pressure regulating valve 130, 230 are respectively integrated in the air outlet 112, 212. To bypass the fuel cell stack 100, 200, the air supply 111, 211 and the air outlet 112, 212 can be coupled via a bypass 118, 218, respectively, to an integrated bypass valve 119, 219.

[0030] Fresh anode gas or hydrogen and recirculated anode gas are supplied to the anodes 120, 220 of both fuel cell stacks 100, 200 via anode circuits 121, 221, respectively. Recirculation is generated passively using jet pumps 124, 224 and actively using blowers 123, 223, respectively. Nitrogen diffusing from the cathode side to the anode side accumulates in the recirculated anode gas over time, so purge valves 122, 222 are provided in the anode circuits 121, 221, respectively. By opening the purge valves 122, 222, the nitrogen-containing anode gas is discharged from the anode circuits 121, 221 and diluted by being introduced into the respective air exhaust passages 112, 212 via connecting pipes 132, 232. Since water may additionally accumulate in the recirculated anode gas, a water separator 126, 226 with a container 127, 227 is additionally integrated in the anode circuit 121, 221. The container 127, 227 can be emptied from time to time by opening the respective drain valve 128, 228.

[0031] Heat generated during operation of the fuel cell stacks 100, 200 is removed using cooling circuits 129, 229, respectively.

[0032] The anode circuits 121, 221 of both fuel cell stacks 100, 200 are connected or can be connected to the air outlets 212, 112 of the other fuel cell stack 200, 100, respectively, via separate connecting pipes 2, 4 with integrated shut-off valves 3, 5. In order to passivate or repassivate the anode surfaces, the shut-off valves 3, 5 can then be opened in turn to allow exhaust air to be introduced from the air outlets 112, 212 of one of the fuel cell stacks 100, 200 into the anode circuits 221, 121 of the other fuel cell stack 200, 100, respectively, via the respective connecting pipes 2, 4. In this case, the steps of the method illustrated in FIG. 4, which are described in detail below, can be carried out.

[0033] In step S30, a passivation of the anode surface of the anode 220 of the fuel cell stack 200 is started. To this end, in step S31, the total pressure level in the fuel cell stack 100 is first increased above the total pressure level in the fuel cell stack 200, so that the pressure in the air discharge line 112 upstream of the turbine 131 is equal to or greater than the pressure in the anode circuit 221 of the fuel cell stack 200. Furthermore, in an optional step S32, the oxygen concentration of the exhaust air in the air discharge line 112 may be reduced. Then, in step S33, the shutoff valve 3 is opened, so that due to the pressure difference, the exhaust air flows from the air discharge line 112 through the connecting pipe 2 into the anode circuit 221 of the fuel cell stack 200. In step S34, it is checked whether a certain amount of passivation and / or passivation time, for example 2 seconds, has been achieved. If the result of the check is positive ("yes"), in step S35, the shutoff valve 3 can be closed again. To the extent that step S32 has been performed, the oxygen concentration of the exhaust air in air exhaust passage 112 may be set back to a standard level in step S36. In step S37, the total pressure level in fuel cell stack 100 may be set back to a standard level, so that the method may end in step S38.

[0034] In a corresponding manner, the anode surface of the anode 120 of the first fuel cell stack 100 can be passivated or repassivated via the connecting tube 4 and the shutoff valve 5 . [Explanation of symbols]

[0035] 1. Fuel cell system 2,4 joint tube 3,5 Shut-off valve 100 First fuel cell stack 110,210 Cathode 111,211 Air supply passage 112,212 Air exhaust passage 120,220 Anode 121,221 Anode circuit 122 Purge valve 123 Blower 128 Drain valve 132 Combined pipe 200 Second Fuel Cell Stack

Claims

1. A method for operating a fuel cell system (1) comprising at least one fuel cell stack (100) having a cathode (110) and an anode (120), comprising, during standard operation of the fuel cell system (1), supplying air to the cathode (110) via an air supply (111) and discharging exhaust air leaving the fuel cell stack (100) via an air discharge (112), and supplying hydrogen to the anode (120) via an anode circuit (121), a method for introducing exhaust air from the air exhaust (112) or from an air exhaust (212) of another fuel cell stack (200) into the anode circuit (121) of the anode (120) in order to build up a passivation layer of the anode (120) and / or to repassivate the passivation layer of the anode (120).

2. 2. The method according to claim 1, characterized in that the branched exhaust air is integrated into the anode circuit (121) and introduced through a purge valve (122) and / or a drain valve (128) that is connected to the air exhaust line (112) of the same fuel cell stack (100) via a connecting pipe (130).

3. 3. A method according to claim 1 or 2, characterized in that the pressure in the air discharge (112) is temporarily increased, for example by 20 mbar, compared to the pressure in the anode circuit (121).

4. 2. The method according to claim 1, characterized in that the exhaust air branched off from the air exhaust path (212) of another fuel cell stack (200) is introduced into the anode circuit (121) of the first fuel cell stack (100) via a separate connecting pipe (2) having an integrated shut-off valve (3).

5. 5. The method of claim 4, characterized in that the total pressure level of the other fuel cell stack (200) is temporarily increased relative to that of the first fuel cell stack (100).

6. Method according to any one of the preceding claims, characterized in that the oxygen concentration of the exhaust air in the air exhaust (112, 212) is temporarily reduced.

7. 7. The method according to claim 1, characterized in that during the introduction of the exhaust air into the anode circuit (121), a blower (123) integrated in the anode circuit (121) is operated.

8. A fuel cell system (1) comprising a plurality of fuel cell stacks (100, 200) each having a cathode (110, 210) and an anode (120, 220), the cathodes (110, 210) being coupled to an air supply passage (111, 211) on a supply side and to an air discharge passage (112, 212) on a discharge side, and the anodes (120, 220) being connected to an anode circuit (121, 221), A fuel cell system, characterized in that the air exhaust passage (112, 212) of at least one fuel cell stack (100, 200) can be connected to the anode circuit (221, 121) of another fuel cell stack (200, 100) via a separate connecting pipe (2, 4) having an integrated shut-off valve (3, 5).

Images