Method for operating a fuel cell system, and associated control device

Abstract

The invention relates to a method for operating a fuel cell system comprising at least one fuel cell which has a membrane arranged between two catalyst layers in order to form an anode and a cathode, wherein, during normal operation, hydrogen is supplied to the anode via an anode circuit. The following steps are carried out prior to a system start: - opening at least one discharge valve, for example a drain and / or purge valve, integrated in the anode circuit, - drawing ambient air into the anode circuit and into the anode of the at least one fuel cell via the at least one opened discharge valve, and - removing impurities from the anode-side catalyst layer by way of the drawn-in air. According to the invention, the discharge valve integrated into the anode circuit is opened if the fuel cell is deactivated and the hydrogen concentration in the anode circuit is between 15 and 30 percent by volume. The invention also relates to a control device for carrying out steps of the method according to the invention.

Description

[0001] R.415981

[0002] - 1 -

[0003] Description

[0004] Method for operating a fuel cell system, control unit

[0005] The invention relates to a method for operating a fuel cell system with the features of the preamble of claim 1. The invention further relates to a control unit for carrying out steps of the method.

[0006] The preferred application area is mobile fuel cell systems or fuel cell vehicles.

[0007] State of the art

[0008] Fuel cells are electrochemical energy converters. Hydrogen (H2) and oxygen (O2) can be used as reaction gases. These are converted into electrical energy, water (H2O), and heat by a fuel cell. To increase electrical power, a large number of fuel cells are connected in a stacked arrangement to form a fuel cell stack. The fuel cell stack contains numerous channels through which the individual fuel cells are supplied with the necessary media and through which the depleted media exiting the fuel cells are removed.

[0009] The core of a fuel cell is the membrane electrode assembly (MEA). This comprises a membrane coated on both sides with a catalytic material to form electrodes. The electrodes, or catalyst layers, typically consist of platinum particles deposited on larger carbon particles. While the platinum particles form the catalyst, the carbon phase ensures electron and heat transport. In addition, the catalyst layers are usually permeated with an ionomer to guarantee proton conductivity. (R.415981)

[0010] - 2 -

[0011] When platinum, ionomer and reactant meet, three-phase boundaries are formed, which are necessary for the electrochemical reaction.

[0012] On the anode side, at the three-phase boundaries, catalyzed by the platinum particles, the oxidation of hydrogen to protons and electrons takes place according to the equation H2 = 2H+ + 2e' instead One disadvantage associated with the use of platinum as a catalyst is its very low tolerance to carbon monoxide poisoning, which can occur through the use of impure hydrogen. In this case, carbon monoxide is adsorbed onto the active platinum surfaces, 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.

[0013] In practice, this poisoning mechanism is slowed down by oxygen permeation from the cathode to the anode of the fuel cell. However, this requires that the concentration of impurities in the hydrogen remains within an acceptable range. If this is the case, the oxygen permeation flow is sufficient to oxidize parasitic carbon monoxide in the anode electrode and keep the anode electrode free of carbon monoxide adsorbates. If, however, the impurity concentration exceeds the acceptable range, the oxygen permeation flow is no longer sufficient, resulting in a decrease in the anode's efficiency over time.

[0014] The present invention addresses the objective of minimizing impurities present on the anode electrode in order to increase the performance and service life of the fuel cell. The focus is not only on carbon monoxide adsorbates, but also on other harmful adsorbates or impurities that coat the catalyst surface and thus block the catalytic reaction. Besides carbon monoxide, these include, for example, sulfur and sulfur compounds, hydrocarbons, and / or nitrogen oxides.

[0015] To solve the problem, the method with the features of claim 1 is proposed. Advantageous embodiments are described in the dependent claims. Furthermore, a control unit for executing steps of the method is proposed. R.415981

[0016] - 3 -

[0017] Disclosure of the invention

[0018] A method for operating a fuel cell system is proposed, comprising at least one fuel cell with a membrane arranged between two catalyst layers to form an anode and a cathode, wherein, during normal operation, the anode is supplied with hydrogen via an anode circuit. The following steps are performed before system startup:

[0019] Opening at least one drain valve integrated into the anode circuit, for example a drain and / or purge valve, drawing in ambient air through the at least one open drain valve into the anode circuit and into the anode of the at least one fuel cell and

[0020] Cleaning the anode-side catalyst layer of impurities using the drawn-in air.

[0021] According to the invention, the opening of the drain valve integrated into the anode circuit is carried out when the fuel cell is deactivated and the hydrogen concentration in the anode circuit is between 15 and 30 volume percent.

[0022] The ambient air drawn into the anode, or rather the oxygen it contains, oxidizes the impurities coating the catalyst layer, thus removing them. This occurs at the latest when normal operation resumes, as the impurities are then carried away with the hydrogen. The process of contamination or poisoning of the anode electrode is thereby reversed, increasing the efficiency and lifespan of at least one fuel cell.

[0023] If the contamination or poisoning is caused, for example, by carbon monoxide, which coats the anode-side catalyst layer of platinum particles (C,CO poisoning), the proposed procedure is preceded by the following reaction:

[0024] Pt + CO <-> Pt - CO R.415981

[0025] - 4 -

[0026] The proposed introduction of air or oxygen ("air bleed") into the anode causes the following reaction to take place on the free Pt surfaces of the anode-side catalyst layer:

[0027] 02+ 2 Pt 02- Pt + Pt -> 2 Pt - 0

[0028] Subsequently, the adsorbed CO is chemically oxidized to CO2, thus cleaning the anode electrode:

[0029] Pt - O + Pt - CO -> CO2 + 2 Pt

[0030] Similarly, other impurities on the anode electrode can also be removed using the proposed method.

[0031] A particularly advantageous aspect of the inventive method is that air is drawn in from the environment only once, and the anode is otherwise sealed, so that no new hydrogen is supplied. This distinguishes the method from classic "air bleed" methods, which are a continuous process in which the anode is supplied with fresh hydrogen, and air is continuously added to this hydrogen.

[0032] The inventive method enables recirculation of the gas produced in this way in the anode, which advantageously optimizes the distribution of oxygen on the anode over the entire stack height and thereby ensures the removal of reaction products from the platinum surface.

[0033] The proposed procedure is carried out before the actual system start-up, since at that time there is usually no oxygen on the cathode, but essentially a mixture of hydrogen and nitrogen. This avoids an air-to-air start-up, which is particularly damaging to the cathode-side catalyst layer due to carbon corrosion (C). Therefore, carrying out the proposed procedure before system start-up also protects the cathode-side catalyst layer, as no air reaches the cathode side. R.415981

[0034] - 5 -

[0035] The fuel cell is deactivated before the drain valve is opened, so the process takes place without a load. The fuel cell is switched off and therefore advantageously passive.

[0036] Maintaining a hydrogen concentration of 15 to 30% by volume in the anode circuit before opening the drain valve prevents undesirable side reactions of hydrogen and oxygen, such as potential oxyhydrogen reactions with the associated heat generation in the anode circuit. Instead, direct, electrochemically active CO oxidation can advantageously occur at the anode catalyst. Through the reaction of an initial amount of hydrogen, the excess oxygen is adsorbed onto the platinum catalyst surface and is then directly available for the oxidation of CO to CO2.

[0037] Depending on the balanced pressure difference, the amount of oxygen introduced can be up to 6% by volume in the anode gas, and then reacts down to 0% by volume. With a simultaneously low hydrogen concentration, this advantageously results in a stoichiometric H₂ / O₂ ratio of 2:1 or even slightly lower.

[0038] Preferably, a negative pressure present in the anode circuit and the anode is used to draw in air via the at least one open purge valve. The system enters a negative pressure state, for example, when a so-called "bleed down" is performed during shutdown, in which the oxygen supply to the cathode is interrupted and the oxygen present on the cathode side is consumed. As the system subsequently cools, a negative pressure is created. In addition, some of the hydrogen present on the anode side can react with residual and / or subsequently diffusing oxygen, causing the pressure to drop further in the switched-off state. The negative pressure is therefore already present when the proposed procedure for cleaning the anode electrode is to be carried out before a system start-up. It is essential that the negative pressure in the anode relative to the surroundings exists before the purge / drain valves are opened.This allows ambient air to be drawn in advantageously through pressure equalization. This is in contrast to conventionally designed and actively purged R.415981.

[0039] - 6 -

[0040] With the method proposed here, no additional units for supplying air to the anode are necessary.

[0041] To ensure that the cathode is essentially oxygen-free during the process and that an air-to-air start does not occur, a further development of the invention proposes that, upon prior shutdown of the system, the supply of air to the cathode is interrupted and any oxygen present on the cathode side is consumed. This is known as a bleed-down process. Upon subsequent system start-up, the cathode is then essentially oxygen-free.

[0042] Furthermore, it is proposed that the procedure be carried out at regular intervals, for example, after every few hundred operating hours. Alternatively, it is proposed that the procedure be carried out before every nth system start-up. As already mentioned, there is no oxygen on the cathode before the actual system start-up, so the procedure can be carried out particularly gently. After a short start-up delay, the actual start-up procedure can then be performed, in which the air previously drawn into the anode is removed and replaced by hydrogen. The system can then continue to operate normally. The system then operates with higher efficiency than before the shutdown.

[0043] Furthermore, a control unit is proposed that is configured to execute steps of a method according to the invention. The proposed method can be started using the control unit. For this purpose, a recovery protocol can be stored in the control unit, according to which the method is then carried out. The control unit can be used to actuate and open the at least one drain valve. If a gas supply unit is integrated into the anode circuit, it can be activated using the control unit to draw in air via the open drain valve and / or to distribute the drawn-in air evenly.

[0044] The invention is explained in more detail below with reference to the accompanying drawing. This shows a diagram illustrating the pressure profile in a fuel cell stack with multiple fuel cells in the switched-off state. R.415981

[0045] - 7 -

[0046] Detailed description of the drawing

[0047] The proposed method for operating a fuel cell system involves drawing air into the anode before system startup to oxidize and remove impurities from the anode electrode. This is intended to increase the efficiency and lifespan of the fuel cells. The air intake can be achieved passively and / or actively.

[0048] For passive air intake, a negative pressure can be used, which arises when the system is switched off, among other things, because the system cools down and / or any water vapor present condenses and / or residual oxygen reacts with hydrogen to form water. As illustrated by way of example in the figure, the pressure drops very sharply in the switched-off state, especially within the first 10 to 15 hours. After this time, a pressure increase can occur due to leakage, as air is drawn in. However, the pressure level present before the switch-off is not reached again, so that a negative pressure still exists in the system when it is subsequently started up. This can then be used to carry out the method according to the invention.

Claims

R.415981 - 8 - Claims 1. Method for operating a fuel cell system comprising at least one fuel cell with a membrane arranged between two catalyst layers to form an anode and a cathode, wherein in normal operation the anode is supplied with hydrogen via an anode circuit, wherein the following steps are performed before a system start-up: Opening at least one drain valve integrated into the anode circuit, for example a drain and / or purge valve, Drawing in ambient air through the at least one open drain valve into the anode circuit and into the anode of the at least one fuel cell, and Removing impurities from the anode-side catalyst layer by means of the drawn-in air, characterized in that the drain valve integrated into the anode circuit is opened when the fuel cell is deactivated and the hydrogen concentration in the anode circuit is between 15 and 30 volume percent.

2. Method according to claim 1, characterized in that a negative pressure present in the anode circuit and in the anode is used to draw in the air via the at least one open drain valve.

3. Method according to claim 1 or 2, characterized in that, in the event of a prior shutdown of the system, the supply of air to the cathode is interrupted and oxygen present on the cathode side is consumed.

4. Method according to one of the preceding claims, characterized in that the method is carried out after several hundred operating hours or before each nth system start. R.415981 - 9 - 5. Control unit configured to perform steps of a method according to any of the preceding claims.

Images