Method for operating a fuel cell system, controller

Abstract

The invention relates to a method for operating a fuel cell system comprising a fuel cell stack having an anode and a cathode, in which method the following steps are carried out for oxidizing contaminants, in particular adsorbates, present in the anode: S1) interrupting the supply of air to the cathode of the fuel cell stack, and reducing the cell voltage by electrochemically reducing the remaining oxygen present in the cathode; s2) interrupting the supply of hydrogen to the anode and electrochemically pumping the remaining hydrogen present in the anode to the cathode; and S3) oxidizing the contaminants by increasing the anode potential. The invention also relates to a controller for carrying out the steps of the method according to the invention.

Description

Technical Field

[0001] The invention relates to a method for operating a fuel cell system and also to a controller for executing the steps of the method.

[0002] Preferred areas of application are mobile fuel cell systems or fuel cell vehicles. Background Art

[0003] A fuel cell is an electrochemical energy converter. In particular, hydrogen (H2) and oxygen (O2) can be used as reactant gases. They are converted into electrical energy, water (H2O) and heat by means of a fuel cell. In practice, a large number of fuel cells are connected to form a fuel cell stack or a stack to increase the electrical power. A plurality of channels runs through the stack, specifically supply channels for supplying the required media to the individual fuel cells and removal channels for carrying away the consumed or exhausted media.

[0004] The core of a fuel cell is a membrane electrode assembly (MEA). It includes a membrane coated with a catalytic material on both sides to form electrodes. The electrode or catalyst layer is usually composed of platinum particles applied to larger carbon particles. The platinum particles constitute the catalyst, while the carbon phase (Kohlenstoff phase) is responsible for electron transport and heat transport. In addition, the catalyst layer is usually traversed by an ionomer to ensure proton conductivity. Due to the collision of the platinum, ionomer and reactants, a three-phase boundary is generated, which is necessary for the electrochemical reaction.

[0005] On the anode side, hydrogen is catalytically oxidized by platinum particles at the three-phase boundary into protons and electrons according to the following equation: H2 = 2H + +2e - A disadvantage when using platinum as a catalyst is the very low tolerance to poisoning by carbon monoxide, which can be caused by the use of impure hydrogen. In this case, carbon monoxide is adsorbed on the active platinum surface, which not only blocks the hydrogen reaction on the platinum sites occupied by carbon monoxide, but also reduces the activity of the free platinum sites.

[0006] In practice, this poisoning mechanism is mitigated by oxygen permeation from the cathode side to the anode side of the fuel cell. However, this assumes that the concentration of hydrogen contaminants is within an acceptable range. If this is the case, the oxygen permeation flow is sufficient to oxidize the parasitic carbon monoxide in the anode electrode and keep the anode free of carbon monoxide adsorbates. However, if the concentration of contaminants exceeds the acceptable range, the oxygen permeation flow is no longer sufficient, which causes the performance of the anode to degrade over time.

[0007] In order to prevent this, catalysts can be used on the anode side which are more tolerant to carbon monoxide poisoning than platinum. These catalysts include binary platinum alloys such as PtRu, PtSn, PtRh, PtMo, PtNi, PtFe, PtIr and other ternary alloys. By using corresponding catalysts, the oxidation potential can be lowered and / or the adsorption capacity for carbon monoxide can be reduced. The actual cleaning step is then carried out by increasing the potential on the anode side, for example by load pulses. However, it has been demonstrated here that other problems may arise, such as physicochemical instabilities of the fuel cell and the resulting faster degradation, reduced performance and increased costs.

[0008] Alternatively, ultrapure hydrogen may be used. However, this solution is not feasible due to high costs and / or the unavailability of ultrapure hydrogen. Summary of the invention

[0009] The present invention is directed to the task of overcoming or at least minimizing the above-mentioned disadvantages in the operation of a fuel cell system. The focus here is not only on carbon monoxide adsorbates, but also on other harmful adsorbates which may occupy the catalyst surface and thus hinder the catalytic reaction. In addition to carbon monoxide, these harmful adsorbates also include sulfur and sulfur compounds, hydrocarbons and / or nitrogen oxides.

[0010] In order to achieve this object, a method for operating a fuel cell system is proposed. Advantageous embodiments can be derived from the preferred embodiments. In addition, a controller for executing the steps of the method is proposed.

[0011] A method for operating a fuel cell system comprising a fuel cell stack having an anode and a cathode is proposed. In the method, the following steps are performed for oxidizing pollutants, in particular adsorbates, present in the anode:

[0012] S1 interrupts the air supply to the cathode of the fuel cell stack and reduces the cell voltage by electrochemically reducing the residual oxygen present in the cathode.

[0013] S2 interrupts the hydrogen supply to the anode and electrochemically pumps the remaining hydrogen present in the anode to the cathode, and

[0014] S3 oxidizes pollutants by increasing the anode potential.

[0015] The proposed method can be used to oxidize not only carbon monoxide, but also other parasitic adsorbates that occupy the platinum particles or platinum alloy particles contained in the anode-side catalyst layer and remove carbon monoxide and other parasitic adsorbates from the anode. This removal is carried out by means of flushing, specifically at the latest when the fuel cell system is restored to normal operation.

[0016] No additional components are required to carry out the method. Furthermore, platinum or platinum alloys can also be used as catalysts. Furthermore, there is no need to be concerned about the purity of the available hydrogen.

[0017] An important method step is to increase the anode potential in step S3. The anode potential to be reached may be different depending on the pollutant to be oxidized. For example, carbon monoxide adsorbed on platinum can be oxidized at a potential between 0.6 V and 0.8 V. For sulfur compounds and nitrogen oxides, potentials of more than 0.8 V or even more than 0.9 V are required.

[0018] Therefore, preferably, in step S3, the anode potential is increased to 0.4 V to 1.5 V, preferably to 0.6 V to 1.0 V, and particularly preferably to 0.8 V, compared to the cathode potential.

[0019] All potentials mentioned are given relative to the normal hydrogen electrode. The voltages given always relate to a single cell and are defined as the cathode potential minus the anode potential (with respect to the configuration in normal operation of the fuel cell).

[0020] Such high anode potentials cannot normally be achieved without further measures, since the equilibrium potential of the anode half-cell is approximately 0 V.

[0021] Therefore, it is further preferred that the anode potential is increased in step S3 by a potential jump, a potential ramp or a cyclic potential ramp. In the case of a potential jump, the increase is performed step by step. In the case of a cyclic potential ramp, also known as cyclic voltammetry, the cell voltage is cycled between 0 V and -1.0 V, for example at a rate between 10 mV / s and 500 mV / s in one to four cycles.

[0022] Steps S1 to S3 can be performed at regular time intervals, for example, according to a previously defined recovery protocol. Depending on the contamination of the hydrogen, the intervals can be adapted. The performance of the method at regular time intervals presupposes the presence of a buffer battery or an additional fuel cell stack, the capacity or power capability of which is sufficient to replace the electrical power of the fuel cell stack involved during the performance of the method.

[0023] If there is no buffer battery or if there is no additional fuel cell stack to cover the power demand, steps S1 to S3 can also be performed when the fuel cell system is shut down. This is because when shutting down, a so-called "oxygen bleed" (also called "air cut-off" in English: Air-Starve) usually occurs, in which the voltage of the fuel cell stack is reduced by electrochemically reducing the residual oxygen in the cathode. Accordingly, when shutting down, the measures given in step S1 of the proposed method have already been performed, so that only steps S2 and S3 need to be performed next.

[0024] Steps S1 and S2 do not necessarily have to be performed successively. They can also be performed simultaneously or overlap in time.

[0025] In order to interrupt the air supply in step S1 , valves are preferably first closed which are arranged in the air system of the fuel cell system in the region of the inlet and outlet of the fuel cell stack. The cell voltage can then be reduced.

[0026] The cell voltage is preferably forced to be reduced in step S1 by current tapping. The current can also be fed into vehicle systems and / or batteries, in particular buffer batteries of fuel cell systems.

[0027] At the latest after the cell voltage has been reduced, the hydrogen supply is interrupted to carry out step S2. Hydrogen is still present in the anode, while only nitrogen and water are present in the cathode. The hydrogen present in the anode is then electrochemically pumped to the cathode side.

[0028] Preferably, in step S2, electrochemical pumping is achieved by applying a current and / or a voltage. As a result, the voltage becomes negative. The cathode constitutes a reference electrode and a counter electrode through the hydrogen present in the cathode. Here, the minimum amount of hydrogen on the cathode side is sufficient to constitute a good reference electrode or counter electrode. This means that it is not necessary to pump all the hydrogen still present on the anode side to the cathode side. However, with the remaining amount of hydrogen on the anode side, the current increases in step S3 of the method. Therefore, a particularly preferred embodiment is to reduce the remaining amount of hydrogen by interrupting the hydrogen supply at an appropriate time during step 1. Here, the time point is selected so that the amount of hydrogen in the anode path is still high enough to ensure complete conversion of the remaining amount of oxygen.

[0029] Then, in step S3 , the anode potential is increased, for example by cyclic voltammetry, by one or more cell voltage ramps or by one or more cell voltage jumps, to oxidize the adsorbate or adsorbates.

[0030] In the simplest embodiment, the voltage of the entire stack is adjusted via the end plates in step S3. Here, it can be ensured by means of individual cell voltage measurement (English: Cell Voltage Monitoring, abbreviation: CVM) that no cell voltage exceeds a given maximum value (e.g. 1V). In an alternative embodiment, in step S3, a controlled voltage is applied to each individual cell in the stack either simultaneously, successively or in groups. In order to tap the differential current of the individual cells, an attachment of an individual cell monitoring system can be used for this purpose.

[0031] Depending on the amount of hydrogen on the cathode side, normal operation of the fuel cell system can continue after step S3 without taking further measures beforehand. This is particularly the case if only a low amount of hydrogen is present on the cathode side.

[0032] Alternatively, the hydrogen present in the cathode may be pumped back to the anode after performing steps S1 to S3 before continuing with normal operation of the fuel cell system.

[0033] During normal operation of the fuel cell system, the oxidized adsorbate is simply flushed out.

[0034] Furthermore, a controller is provided, which is configured to carry out the steps of the method according to the invention. For example, a recovery protocol can be stored in the controller, according to which the method is carried out at regular time intervals or when the fuel cell system is shut down. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Below, the present invention and its advantages are explained in more detail with reference to the accompanying drawings. The accompanying drawings show:

[0036] Figure 1 A flow chart is shown for illustrating a preferred process of the method of the present invention;

[0037] Figure 2 Shown is the cyclic voltammogram of a Pt / C electrode poisoned with carbon monoxide. DETAILED DESCRIPTION

[0038] Figure 1 The process shown in exemplarily includes three steps from S1 to S3.

[0039] In step S1, the air supply to the cathode is first interrupted by closing the valves at the inlet and outlet of the fuel cell stack, thereby leaving a volume of oxygen enclosed therein on the cathode side. Subsequently, the fuel cell is further forced to react by current tapping to reduce oxygen and bring the cell voltage to below 0.2 V, preferably below 0.1 V, particularly preferably below 0.05 V.

[0040] In the following step 2 (which can also be performed simultaneously with step S1 or at least overlapped in time), the hydrogen supply to the anode is first interrupted. Subsequently, the hydrogen present in the anode is brought to the cathode side by electrochemical pumping.

[0041] In step S3 , the parasitic adsorbates are then oxidized by increasing the anodic potential, for example by a potential jump or by a cyclic potential ramp, which is also referred to as cyclic voltammetry.

[0042] Figure 2The graph of is shown by way of example in which the oxidation of carbon monoxide by cyclic voltammetry is performed, wherein curve A is assigned to the first cycle and curve B to the second cycle. Area 1 shows the oxidation peak. Area 2 shows the acquisition of an active platinum surface after the first cycle.

Claims

1. A method for operating a fuel cell system comprising a fuel cell stack having an anode and a cathode, in which the following steps are performed for oxidizing pollutants, in particular adsorbates, present in the anode: S1 interrupts the air supply to the cathode of the fuel cell stack and reduces the cell voltage by electrochemically reducing the remaining oxygen present in the cathode, S2 interrupts the supply of hydrogen to the anode and electrochemically pumps the remaining hydrogen present in the anode to the cathode, and S3 oxidizes the pollutants by increasing the anode potential.

2. The method according to claim 1, It is characterized in that In step S3 , the anode potential is increased to 0.4 V to 1.5 V, preferably to 0.6 V to 1.0 V, particularly preferably to 0.8 V, compared to the cathode potential.

3. The method according to claim 1 or 2, It is characterized in that In step S3 , the anode potential is increased by a potential jump, a potential ramp or a cyclic potential ramp.

4. The method according to claim 1, It is characterized in that Steps S1 to S3 are performed at regular time intervals.

5. The method according to any one of the preceding claims, It is characterized in that Steps S1 to S3 are performed when the fuel cell system is shut down.

6. The method according to any one of the preceding claims, It is characterized in that Step S1 and step S2 are performed simultaneously or overlapping in time.

7. The method according to any one of the preceding claims, It is characterized in that In step S1 , the cell voltage is forcibly reduced by current tapping and current is fed into a vehicle system and / or a battery, in particular a buffer battery of the fuel cell system.

8. The method according to any one of the preceding claims, It is characterized in that In step S2 , electrochemical pumping is achieved by applying current and / or voltage.

9. The method according to any one of the preceding claims, It is characterized in that After carrying out steps S1 to S3 , the hydrogen present in the cathode is pumped back to the anode before continuing with the normal operation of the fuel cell system.

10. A controller configured to perform the steps of the method according to any one of the preceding claims.

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