Method for regenerating fuel cell

Abstract

The invention relates to a method for regenerating a fuel cell (100). The fuel cell (100) has an anode chamber (100b) and a cathode chamber (100a). The fuel cell (100) further comprises an anode-side electrode layer (4) and a cathode-side electrode layer (3). The anode chamber (100b) is in fluid connection with a recirculation circuit (110). The regeneration method is characterized by the following method steps:-stopping the supply of hydrogen from the tank (120) into the anode chamber (100b),-connecting the anode chamber (100b) with the recirculation loop (110) and thus supplying a recycle into the anode chamber (100b),-performing a regeneration step.

Description

Field of Technology

[0001] The present invention relates to a method for regenerating a fuel cell. Background Art

[0002] Within the framework of ensuring environmental mobility, vehicles driven by fuel cells have proven to be particularly suitable alternatives to vehicles driven by internal combustion engines. The energy required for propulsion is obtained here in the form of electrical energy through a controlled reaction between hydrogen and oxygen. For this purpose, for example, a PEM fuel cell has a complex polymer electrode membrane assembly (PEM assembly), with an anode arranged on one side and a cathode arranged on the other side. During the operation of the fuel cell, hydrogen is supplied to the anode through a bipolar plate, while oxygen or air is supplied to the cathode. Since a single fuel cell cannot generate enough electrical energy to drive a motor vehicle, multiple fuel cells are connected in series in a fuel cell stack to be able to provide the required electrical energy.

[0003] Disadvantageously, fuel cells are subject to significant aging and degradation during their service life. The aging and degradation effects are mainly caused by the damage of catalyst particles (such as platinum) in the catalyst layer arranged within the membrane assembly. In addition to irreversible damage, such as the loss of platinum or the aggregation of platinum particles into larger particles, the performance of the fuel cell is particularly reduced due to reversible degradation, such as the formation of platinum oxides or the adsorption of sulfonates on the platinum surface. Furthermore, if such reversible degradation is not reversed, that is, the sulfonates are removed and the platinum oxides are reduced to platinum, thereby preventing the formation of so-called 3D platinum oxides, it may also lead to an exacerbation of irreversible degradation.

[0004] Measures for preventing such aging and degradation are known in the prior art. For example, DE102013227217A1 proposes using hydrogen on the cathode side of the fuel cell to resist the formation of oxides on the catalyst layer.

[0005] The disadvantage here is that the supply of fuel hydrogen for the regeneration operation reduces the amount of hydrogen required for the normal operation of the fuel cell stack, thereby reducing the driving range of the vehicle when applied to an automobile. Summary of the Invention

[0006] The subject matter of the present invention is a method for regenerating a fuel cell. The fuel cell has an anode chamber and a cathode chamber. The fuel cell also includes an anode-side electrode layer and a cathode-side electrode layer. The anode chamber is fluidly connected to a recirculation loop.

[0007] The regeneration method is characterized by the following method steps:

[0008] Stop the supply of hydrogen from the storage tank to the anode chamber,

[0009] - Connect the anode chamber to the recirculation loop and thus supply the recirculated material to the anode chamber,

[0010] - Perform a regeneration step.

[0011] This regeneration method is naturally different from the normal operation of the fuel cell. This difference lies, for example, in that the anode chamber is attached to the recirculation loop while the connection to the storage tank is disconnected; thereby, water, especially the water on the anode side, is discharged from the fuel cell. At the same time, the regeneration step itself forms another difference from the normal operation, that is, it is performed in a hydrogen-saving manner by regenerating only with the hydrogen present in the recirculation loop.

[0012] Preferably, the discharge of water from the anode chamber is assisted by a recirculation compressor arranged in the recirculation loop. Preferably, the pressure in the anode chamber is increased by the recirculation compressor here such that the pressure in the anode chamber is higher than the pressure in the cathode chamber. This also has the further advantage of hindering the diffusion of oxygen or air into the anode chamber.

[0013] Within the regeneration method, one or more regeneration steps can be performed. The regeneration steps can be, for example:

[0014] - Activate the no-load operation of the fuel cell,

[0015] - Feed a continuous negative current or a periodically alternating positive and negative current or a time-varying current with a negative effective value to the fuel cell so as to oxidize the introduced hydrogen at the anode-side electrode layer of the fuel cell and reduce H+ at the cathode-side electrode layer of the fuel cell.

[0016] Thereby, the platinum oxides and sulfonate adsorbents on the cathode-side electrode layer of the membrane electrode assembly are removed particularly effectively. In this way, the efficiency and life of the fuel cell or the battery stack can be improved, in particular. In addition, the improvement of the efficiency and life can be used to save platinum or reduce the size of the cooling system of the fuel cell system.

[0017] This method can be used in particular for passenger cars or lorries. It can also be considered for ships, aircraft or stationary objects. According to the invention, no-load operation can in particular be understood as the state in which no electrical appliances are connected to the battery stack - and thus preferably only the no-load voltage is applied; no-load operation is therefore also a deviation from normal operation. Furthermore, it is obvious that activating the no-load operation of the fuel cell can equally be understood as maintaining the no-load operation if the subject method starts from a non-activated state or no-load mode. Similarly, interrupting the supply of air or oxygen, in particular on the cathode side of the fuel cell, can also be understood as maintaining the interruption of the supply of air or oxygen. Furthermore, it is obvious that oxygen can be supplied either in pure form or in the form of air. Within the framework of the invention, a continuous negative current can in particular be understood as the following current flow: this current flow is directed in a current flow direction opposite to the existing current flow direction (from the anode to the cathode). Accordingly, a periodically alternating positive and negative current can be understood as a current with a periodically changing current flow direction. Applying a periodically alternating positive and negative current can in particular be advantageous in order to alternately generate and remove hydrogen on the cathode so that the hydrogen concentration on the cathode side does not reach a safety-relevant magnitude. Alternatively, during the duration of the regeneration step, a current that varies over time can also be applied to the fuel cell, the effective value (root mean square value) of which is negative during this duration.

[0018] In order to remove all oxygen on the cathode side, in addition to taking measures to prohibit the supply of oxygen, it can also be provided that flushing or pumping is carried out on the cathode side before or during the implementation of the subject method. Furthermore, the supply of hydrogen to the fuel cell stack or the cathode part of the battery stack can also be used to effectively prevent air / air start-up, thereby increasing the service life of the fuel cell system.

[0019] Then preferably, hydrogen is also only guided from the recirculation circuit into the cathode chamber of the fuel cell.

[0020] Other features and details of the invention result from the respective dependent claims, the description and the drawings. Among them, the features and details described in connection with the method of the invention naturally also apply to the system of the invention or the motor vehicle of the invention, and vice versa, so that the disclosures regarding the various aspects of the invention can always be cross-referenced.

[0021] The invention further includes a controller having software for performing one of the methods described above. The controller preferably has a detection device for detecting the current state of the fuel cell, a processing unit for determining the necessity of performing a regeneration step based on the detected current state of the fuel cell, and a control unit for controlling the regeneration method. The detection device can, for example, be configured to early identify platinum oxides or sulfonate adsorbates on the surface of the electrode layer on the cathode side of the fuel cell. The processing unit can also preferably be configured to estimate the aging or degradation of the electrode layer based on the detected platinum oxides or sulfonate adsorbates and to determine measures for eliminating the aging or degradation. The control unit can finally be set to control the determined measures, for example for switching valves.

[0022] The invention further includes a motor vehicle having such a controller, and the corresponding software is stored on the controller.

[0023] Other advantages, features and details of the invention result from the following description which details an embodiment of the invention with reference to the accompanying drawings. The features mentioned in the claims and the description are essential to the invention either individually or in any combination. Description of the Drawings

[0024] The drawings show:

[0025] Figure 1 Schematically shows a cross-section of a fuel cell known from the prior art, where only the main areas are shown.

[0026] Figure 2 Schematically shows a fuel cell system having a recirculation loop, where only the main areas are shown.

[0027] Figure 3 Schematically shows another fuel cell system having a recirculation loop, where only the main areas are shown. Detailed Description of the Embodiment

[0028] Figure 1 Schematically shows a fuel cell 100 known from the prior art, where only the main areas are shown. The fuel cell 100 has a membrane 2, in particular a polymer electrolyte membrane. On one side of the membrane 2, a cathode chamber 100a is constructed, and on the other side, an anode chamber 100b is constructed.

[0029] In the cathode chamber 100a, an electrode layer 3, a diffusion layer 5, and a distribution plate 7 are arranged pointing outwards from the membrane 2 - i.e., in the normal direction or the stack direction z. Similarly, in the anode chamber 100b, an electrode layer 4, a diffusion layer 6, and a distribution plate 8 are arranged pointing outwards from the membrane 2. The membrane 2 and the two electrode layers 3, 4 form a membrane electrode assembly 1. In addition, the two diffusion layers 5, 6 can also be part of the membrane electrode assembly 1.

[0030] The distribution plates 7, 8 have channels 11 for supplying gases such as air in the cathode chamber 100a and hydrogen in the anode chamber 100b to the diffusion layers 5, 6. The channels 11 in the cathode chamber 100a form a cathode flow field 11a, and the channels 11 in the anode chamber 100b form an anode flow field 11b. The diffusion layers 5, 6 are generally made of carbon fiber non-woven fabric on the channel side, i.e., facing the distribution plates 7, 8, and are made of a microporous particle layer on the electrode side, i.e., facing the electrode layers 3, 4.

[0031] The distribution plates 7, 8 have channels 11 and thus also implicitly have partitions 12 adjacent to the channels 11. The bottom sides of these partitions 12 thus form contact surfaces 13 between the respective distribution plates 7, 8 and the diffusion layers 5, 6 located below them.

[0032] Generally, the cathode-side distribution plate 7 and the anode-side distribution plate 8 are different from each other; advantageously, the cathode-side distribution plate 7 of a fuel cell 100 is fixedly connected to the anode-side distribution plate 8 adjacent to the fuel cell, for example, by a welding connection, so as to be combined into a bipolar plate.

[0033] In Figure 2 it shows a fuel cell system 200 having a fuel cell 100 with an anode outlet 102 and an anode inlet 101. Here, the fuel cell 100 can also be understood as a fuel cell stack. The fuel cell system 200 has an anode line 20 with an anode inlet 101 and an anode outlet 102. The anode chamber 100b of the fuel cell 100 is fluidly connected to the anode inlet 101 and the anode outlet 102. The anode outlet 102 can also be connected to the anode inlet 101 via a recirculation valve 140 by means of a recirculation loop 110. Here, the recirculation valve 140 is preferably implemented as a three-way valve. In addition, a recirculation compressor 111 is preferably arranged in the recirculation loop 110. The anode line 20 is connected to a storage tank 120, and this connection can be opened and closed by means of a valve 130.

[0034] During the operation of the fuel cell 100, especially hydrogen flows into the anode inlet 101 and is distributed in the anode chamber 100b of the fuel cell 100 through the anode side of the distribution plate 8. For the constant and uniform operation of the fuel cell 100, it is preferably to supply at least the amount of hydrogen required to generate the desired current intensity. More preferably, a larger amount of hydrogen is supplied so that the excess / unconverted hydrogen flows out of the anode chamber 100b again to the anode outlet 102 after passing through the anode side of the distribution plate 8 of the fuel cell 100 and is finally re-supplied to the anode inlet 101 through the recirculation loop 110. The volume flowing through the recirculation loop 110 is called the recirculant.

[0035] On the cathode side, air-oxygen flows into the cathode inlet, passes through the cathode chamber 100a of the fuel cell 100, reacts with hydrogen or with protons diffusing through the membrane 2, and the unreacted / excess air-oxygen flows out again from the cathode outlet (not shown).

[0036] Unfortunately, the fuel cell 100 is subject to significant aging and degradation during its service life. The aging and degradation effects are mainly caused by the damage of the platinum particles in the electrode layers 3 and 4. In addition to irreversible damage, such as the loss of platinum or the aggregation of platinum particles into larger particles, the performance of the fuel cell 100 may be reduced especially due to reversible degradation, such as the formation of platinum oxides or the adsorption of sulfonates on the platinum surface. Moreover, if such reversible degradation is not reversed again, that is, the sulfonates are removed and the platinum oxides are reduced to platinum, thereby preventing the formation of so-called 3D platinum oxides, it may also lead to an exacerbation of irreversible degradation.

[0037] Therefore, reversible degradation can be reversed by a regeneration method with one or more regeneration steps. Examples of such regeneration methods are as follows:

[0038] Activate the no-load operation of the fuel cell system 200,

[0039] Interrupt the oxygen supply to the fuel cell 100,

[0040] Supply hydrogen to the fuel cell 100,

[0041] Feed a continuous negative current or a periodically alternating positive and negative current into the fuel cell 100 in order to oxidize the introduced hydrogen at the anode-side electrode layer 4 of the fuel cell 100 and reduce H+ at the cathode-side electrode layer 3.

[0042] According to the invention, such a regeneration method is now no longer carried out with hydrogen from the storage tank 120, but only with the recycle of the recirculation loop 110, preferably with the hydrogen contained therein. Therefore, the method for regenerating the fuel cell 100 has the following method steps:

[0043] Stop the hydrogen supply from the storage tank 120 to the anode chamber 100b,

[0044] Connect the anode chamber 100b to the recirculation loop 110 and thus supply the recycle to the anode chamber 100b,

[0045] Execute the regeneration step.

[0046] Thereby, the hydrogen consumption of the regeneration method is reduced, and additionally water is discharged from the anode line 20.

[0047] An embodiment of a regeneration method for a fuel cell 100 according to the invention provides the following method steps:

[0048] Activate the no-load operation of the fuel cell system 200,

[0049] Interrupt the oxygen supply to the fuel cell 100,

[0050] Stop the hydrogen supply from the storage tank 120 to the anode chamber 100b,

[0051] Connect the anode chamber 100b to the recirculation circuit 110 and thus supply the recirculated material to the anode chamber 100b,

[0052] Feed a continuous negative current or a periodically alternating positive and negative current or a time-varying current with a negative effective value to the fuel cell 100, so as to oxidize the introduced hydrogen at the anode-side electrode layer 4 and reduce H+ at the cathode-side electrode layer 3.

[0053] In an improved embodiment of the present invention, the recirculated material is compressed by means of the recirculation compressor 111, so that the pressure in the anode chamber 100b is greater than the pressure in the cathode chamber 100a. Thereby, the diffusion of gases, especially oxygen, into the anode chamber 100b is hindered.

[0054] In Figure 3 is shown a fuel cell system 200 similar to Figure 2 . In Figure 3 's embodiment, the cathode chamber 100a of the fuel cell 100 can be connected to the anode outlet 102. Thus, the recirculation circuit 110 can extend through the cathode chamber 100a. The connection between the cathode chamber 100a and the anode outlet 102 is preferably opened and closed by means of the recirculation valve 140. Thereby, the hydrogen used for the regeneration step can be brought into contact with the cathode-side electrode layer 3.

Claims

1. A method for regenerating a fuel cell (100), wherein, The fuel cell (100) has an anode chamber (100b) and a cathode chamber (100a), wherein the fuel cell (100) has an anode-side electrode layer (4) and a cathode-side electrode layer (3), and wherein the anode chamber (100b) is in fluid connection with a recirculation circuit (110). It is characterized by the following method steps: - Stop the supply of hydrogen from the storage tank (120) to the anode chamber (100b). - Connect the anode chamber (100b) to the recirculation circuit (110) and thus supply the recirculated material to the anode chamber (100b). - Perform a regeneration step.

2. The method according to claim 1, wherein Interrupt the supply of hydrogen from the storage tank (120) by means of a valve (130).

3. The method according to claim 1 or 2, characterized in that, Connect the anode chamber (100b) to the recirculation circuit (110) by means of a recirculation valve (140).

4. The method according to any one of claims 1 to 3, characterized in that, Connect the cathode chamber (100a) to the anode outlet (102) by means of a recirculation valve (140).

5. The method according to any one of claims 1 to 4, characterized in that, The recirculation circuit (110) has a recirculation compressor (111).

6. The method according to claim 5, wherein Increase the pressure in the anode chamber (100b) by means of the recirculation compressor (111) such that the pressure in the anode chamber (100b) is higher than the pressure in the cathode chamber (100a).

7. The method according to any one of claims 1 to 6, wherein, The fuel cell (100) is arranged in a fuel cell system (200), and is characterized by the following regeneration steps: - Activate the no-load operation of the fuel cell system (200). - Feed a continuous negative current to the fuel cell (100) in order to oxidize the introduced hydrogen at the anode-side electrode layer (4) of the fuel cell (100) and reduce H+ at the cathode-side electrode layer (3) of the fuel cell (100).

8. The method according to any one of claims 1 to 6, wherein, The fuel cell (100) is arranged in a fuel cell system (200), and is characterized by the following regeneration steps: - Activate the no-load operation of the fuel cell system (200). - Feed a periodically alternating positive and negative current to the fuel cell (100) in order to oxidize the introduced hydrogen at the anode-side electrode layer (4) of the fuel cell (100) and reduce H+ at the cathode-side electrode layer (3) of the fuel cell (100).

9. The method according to any one of claims 1 to 6, wherein The fuel cell (100) is arranged in a fuel cell system (200), and is characterized by the following regeneration steps: - Activate the no-load operation of the fuel cell system (200). - Feed a current with a negative root mean square value that varies with time to the fuel cell (100) in order to oxidize the introduced hydrogen at the anode-side electrode layer (4) of the fuel cell (100) and reduce H+ at the cathode-side electrode layer (3) of the fuel cell (100).

10. A controller having software for performing the method according to any one of claims 1 to 9.

11. A motor vehicle having the controller according to claim 10.

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