Method for operating an electrochemical stack

Abstract

A method for operating an electrochemical stack (10) comprising a plurality of electrochemical cells (1), each cell having an anode compartment (2) with an anode electrode (6) and a cathode compartment (3) with a cathode electrode (7), wherein the anode compartment (2) and the cathode compartment (3) are separated from each other by a semipermeable membrane (8). An electrical voltage is present between the anode electrode (6) and the cathode electrode (7) during operation, with the electrochemical cells (1) being connected in series. A cell voltage monitoring unit (17) is connected to the electrochemical cells (1). The method is characterized by - Measuring electrical voltages U i using the cell voltage monitoring system (17) of n electrochemical cells connected in series, where n is greater than or equal to 2, - Comparing the measured voltages U i with a maximum voltage U exp,nand a minimum voltage U min,n , where the maximum voltage U exp,n n times the maximum possible cell voltage of a single electrochemical cell U exp is and U min,n the (n-1) times the maximum possible cell voltage of a single cell (1) plus a lower voltage limit U Limit , where U Limit the smallest cell voltage down to which operation of a single electrochemical cell (1) is to be carried out, - Output an error message if at least one of the measured voltages U i smaller than U min,n or larger than U exp,n is.

Description

[0001] The invention relates to a method for operating an electrochemical stack comprising a plurality of electrochemical cells, for example a fuel cell stack. State of the art

[0002] Fuel cells consist of a cathode compartment and an anode compartment as chemical reaction chambers, which can be filled with various gases, such as hydrogen on the one hand and oxygen or air on the other. The two reaction chambers are separated by a semipermeable membrane, and each contains an electrode. Redox reactions occur in the reaction chambers due to the two gases, generating electrical energy that can be tapped at the electrodes. Fuel cells are used in various stationary, portable, and mobile applications, for example, in homes for power generation, in motor vehicles, in rail transport, in aviation, and on ships. Since a single electrochemical fuel cell generates only a low voltage, fuel cell units are used, in which a large number of fuel cells are stacked on top of each other to form a stack.The voltage and power of the fuel cells add up, so that even consumers with high power requirements can be supplied.

[0003] For a long service life and for safety reasons, it is important that the individual fuel cells remain within a specific voltage range. The expected maximum voltage of a cell depends on the operating point and varies slightly from cell to cell. However, significantly lower voltages can occur during operation under unfavorable operating conditions or in the event of a fault. A voltage that is too low or even negative must be avoided at all costs, as this can lead to irreversible damage to the cell's membrane and / or catalytic coatings. Therefore, it is necessary to continuously monitor the voltage of all cells to detect malfunctions in a timely manner and to initiate countermeasures. Such a device is called a cell voltage monitoring device, or CVM (cell voltage monitoring).Since fuel cell stacks sometimes contain a very large number of individual cells, each individual fuel cell must be contacted and each voltage monitored. This is complex, correspondingly expensive, and generates a high data stream that must be constantly analyzed. Advantages of the invention

[0004] The method according to the invention makes it possible to monitor the cell voltage of a large number of fuel cells in a fuel cell stack or other electrochemical cell stack without having to contact and monitor the voltage of each individual fuel cell or electrochemical cell. The method according to the invention is applied to an electrochemical stack comprising a plurality of electrochemical cells, each having an anode compartment with an anode electrode and a cathode compartment with a cathode electrode, the anode compartment and the cathode compartment being separated from each other by a semipermeable membrane. The electrochemical cells are connected in series and connected to a cell voltage monitoring system. The cell voltage monitoring system measures the electrical voltages U. iof at least n electrochemical cells connected in series, where n is greater than or equal to two. The individual electrical voltages U i are supplied with a voltage (U) expected at this operating point according to the UI characteristic curve. exp,n ) and a minimum voltage (U min,n ) compared, where the expected voltage U exp,n n times the individual expected voltage U exp is and U min,n (n-1) times the expected voltage plus a minimum limiting voltage U Limit , where the minimum limiting voltage U Limit The lowest cell voltage at which a single electrochemical cell should operate. If one of the measured voltages U i smaller than U min,n An error message will be displayed. At least a warning message can also be displayed if the voltage U i significantly above U exp,nA slight exceedance of this expected maximum voltage is, however, generally not a reason to intervene.

[0005] The method according to the invention can be applied in particular to a fuel cell stack, but optionally also to other electrochemical stacks. The method according to the invention is described below by way of example using a fuel cell stack.

[0006] A fuel cell generates an expected maximum voltage between the electrodes, which depends on the operating point (UI characteristic) and is typically around 0.7 to 0.8 V, with the voltage decreasing as the current increases. If less hydrogen or oxygen is available, the temperature is not optimal, or the cell is already aged, the voltage may be lower. However, the voltage should not drop too low, as this can cause irreversible damage to the membrane and electrodes; in particular, the voltage should not become negative.

[0007] According to the inventive method, for example, two electrochemical cells are measured together, the voltage values ​​being added due to the series connection of the electrochemical cells. Under optimal operating conditions, the expected voltage at a single electrochemical cell is U. expThe lowest voltage that is still tolerable in an electrochemical cell is called U. Limit If this is designated, then the measured total voltage of the n=2 cells should always be above U. min,n = U exp + U Limit This ensures that each of the electrochemical cells has a voltage greater than U. Limit exhibits no abnormalities and damage to the cells is ruled out.

[0008] Each individual electrochemical cell has a UI characteristic curve, where the expected voltage U exp The voltage depends on the respective current or current density through the membrane. The UI characteristic curve can be determined experimentally. However, when evaluating the voltages, it must be taken into account that the UI characteristic curve changes over time, with the voltage decreasing slightly for a given current or current density. In a further development of the invention, the expected voltage U is used to determine the expected voltage.exp The UI characteristic curve used is regularly adjusted. The modified UI characteristic curve can either be regularly remeasured or determined as a function of operating time through modeling and stored in a map in the control unit.

[0009] Instead of just a single minimal voltage U Limit To prevent damage to the fuel cells, a second limit voltage U can also be applied. Limit,2 This must be taken into account. If the voltage at a cell drops to this second value U Limit,2 , which is higher than the minimum limiting voltage U Limit While no damage to a fuel cell is expected at this point, the cell voltage monitoring unit can already issue a (pre-)warning, allowing countermeasures to be prepared. This warning is issued accordingly when monitoring n cells connected in series at U min,n,2 = (n-1) · U exp + U Limit,2output min,n = (n-1) · U exp + U Limit , as before, an error message or alarm will be issued and, if necessary, an emergency shutdown of the electrochemical stack will be initiated to prevent permanent damage to the stack even without external intervention. drawing

[0010] The drawing shows an electrochemical cell, an electrochemical cell stack, and diagrams illustrating the method according to the invention. Fig. 1 a schematic representation of a single fuel cell, Fig. 2 a schematic representation of a fuel cell stack, Fig. 3. A UI characteristic curve of a fuel cell at the beginning and end of its lifetime, showing the limit voltages and Fig. 4 UI characteristic curves of two cells connected in series with lower voltage limits in the new state of the cells and at the end of their service life. Description of the exemplary implementations

[0011] To illustrate the basic principle, in Fig. Figure 1 shows a schematic representation of a single fuel cell 1. A single fuel cell 1 consists of two chemical reaction compartments: an anode compartment 2, which includes gas channels 2a, a porous transport layer 4 (PTL), and a catalytic layer 6, and a cathode compartment 3, which also includes gas channels 3a, a porous transport layer 5, and a catalytic layer 7. The two catalytic layers 6 and 7 are applied to both sides of a membrane 8, which separates the cathode compartment 3 from the anode compartment 2. Together with the membrane, the catalytic layers 6 and 7 form the so-called membrane electrode assembly 13 (MEA). The membrane 8 separates the anode compartment 2 from the cathode compartment 3 and is semipermeable, allowing only certain ions to pass through.The anode compartment 2 is filled with hydrogen gas via the gas channels 2a, and the cathode compartment 3 with oxygen or air via the gas channels 3a, each optionally moistened with water, since the membrane 8 only exhibits the required ionic conductivity in a moist state. The bipolar plate 14 (BPP) separates the anode compartment 2 with its gas channels 2a from the cathode compartment 3 with the gas channels 3a of the next cell 1, which is located in . Fig. The first cell 1 is connected directly above it. Similarly, the next cell connects below cell 1 in the same way. The bipolar plates 14 also form the voltage tap for each individual cell 1.

[0012] The porous transport layer 4 (PTL) arranged in the anode compartment 2 serves to homogenize the gas flow to the catalytic coating 6. The H₂ molecules are split and ionized at the catalytic coating 6. In a so-called PEM fuel cell (proton exchange membrane), the semipermeable membrane 8 is selective for H₂. + -ions permeable. The H formed on the catalytic coating 6 in the anode compartment 2 + Ions move through the membrane 8 and thus reach the catalytic layer 7 in the cathode compartment 3, where they recombine with oxygen to form water (H2O), which is drawn off from the cathode compartment 3. This process generates an electrical voltage U between the bipolar plates 14, which can be utilized.

[0013] To increase the overall voltage and boost the power output, many cells 1 are typically combined to form a fuel cell stack or fuel cell stack 10, with the individual fuel cells 1 being electrically connected in series so that their voltages add up. This is already in the Fig. 1 at the upper end is indicated by another fuel cell 1'. The bipolar plates 14 form the support structure of the stack 10 and are usually made of metal or a composite material. The fuel cell stack 10 is built up by alternately stacking bipolar plates 14 on the one hand and PTLs 4, 5 and MEAs 13 on the other (PTL and MEA are simplified here as follows): Fig. Figure 2 is shown schematically. The upper and lower ends of the fuel cell stack 10 are formed by a first clamping plate 11 and a second clamping plate 12, which are pressed against each other for sealing by a device not shown here. For the sake of clarity, the Fig. Figure 2 shows only a few cells. In reality, a significantly larger number of cells (1) are usually stacked on top of each other, for example 100 or 200 cells, to obtain a higher voltage and more power.

[0014] If the fuel cell stack is supplied with sufficient oxygen and hydrogen, the individual fuel cells 1 deliver a voltage U as expected according to the UI characteristic curve at this operating point. exp , which is, for example, 0.8 V. The voltage U depends on this. exp depending on the applied current strength, as Fig. Figure 3 is shown in a diagram. The UI characteristic curve 20 indicates the behavior of voltage U and current I in the new condition (“begin of life”, BOL). As the operating time progresses, the UI characteristic curve changes and the voltage decreases slightly for a given current I, until finally, at the end of the service life (“end of life”, EOL), the EOL UI characteristic curve 21 is reached.

[0015] The voltage of a single fuel cell can drop significantly below this voltage due to insufficient supply of H2 or O2, aging, or suboptimal temperature. However, if the voltage of a single fuel cell falls below a certain lower limit voltage U, the system will be affected. Limit , irreversible damage to the cell must be expected. This limiting voltage U Limit Therefore, it must not fall below a certain level during operation and amounts, for example, to U Limit = 0 V. The voltage U i,single cellThe individual fuel cell is therefore: U Limit <Ui,single cell≤Uexp

[0016] It should be noted that the expected voltage U exp is a function of the current I, the temperature T, the pressure p in the stack and other physical quantities, and can also vary slightly from cell to cell. Uexp=Uexp(I,T,p,…)

[0017] Since the UI characteristic curve – as already mentioned – decreases over the service life, the voltage window must be continuously adjusted. The respective UI characteristic curve can be re-determined by measurements at regular intervals. However, aging can also be taken into account through modeling by plotting the change in the UI curve against the service life t. Lebensdauer is stored in a characteristic map, so that a measurement on the electrochemical cells is no longer necessary: Uexp=Uexp(tlifetime,I,T,p,…)

[0018] As in Fig. Figure 2 shows that the electrical voltage at the individual electrochemical cells 1 is monitored by a cell voltage monitoring unit 17. Voltage taps 15 are brought out from the bipolar plates 14 and can be measured between the individual voltage taps 15. The individual voltage values ​​are evaluated and, if necessary, stored by a control unit (not shown here). To reduce costs and data flow, however, the voltage is not measured at each individual cell, but at n electrochemical cells connected in series, shown here as an example with n=2 cells. These voltages (U1, U2, U3, ..., U) i These voltages are continuously measured and recorded, with each of these voltages being the sum of the voltages of n fuel cells (here n=2). These voltages U i Therefore, in normal operation, a maximum of U exp,n = n·U exp, i.e., the n-fold expected voltage U exp , which occurs in a single cell.

[0019] To detect in time if one of the cells 1 has a voltage that is too low below U Limit The voltage across n fuel cells must not be less than (n-1) times the expected voltage of a single cell U. exp plus the lower limit voltage U Limit be: Ui≥(n−1)⋅Uexp+ULimit

[0020] Is the measured voltage U i If the voltage in the n cells is above this voltage, it is ensured that the voltage in each individual cell is above the lower limit voltage U. Limit lies. This is in Fig. Figure 4 shows the UI characteristic curves “begin of life” (BOL) 20 and “end of life” (EOL) 21, which show the maximum voltage of here n=2 cells U exp,nThe acceptable voltages across any two individual cells must therefore be between twice the expected voltage when new. Uexp,n=2⋅Uexp and the minimum voltage Umin,n=(2−1)⋅Uexp+ULimit=Uexp+ULimit move, taking into account the state of aging. In its new state, the expected stress limit is Uexp,n,BOL=2⋅Uexp,BOL and the minimum voltage limit Umin,n,BOL=Uexp,BOL+ULimit, Accordingly, towards the end of life, the expected stress limit Uexp,n,EOL=2⋅Uexp,EOL and the minimum voltage limit Umin,n,EOL=Uexp,EOL+ULimit.

[0021] The voltage window is therefore between the maximum BOL or EOL voltage as the upper limit and the respective lower limit U. min,n,BOL / EOL This limiting voltage U min,n,BOL in new condition Fig. 4 with 23, the lower limit voltage U reduced accordingly due to aging min,n,EOL at 24.

[0022] As already mentioned, the stress limits can be tracked through modeling or measurement. However, a simplified model can also be used to minimize effort and avoid tracking the UI characteristics. In this case, the upper stress limit U is... exp,n,EOL used, which corresponds to the UI characteristic curve 21 in Fig. 4 corresponds to the lower limit, whereby only a significant exceedance of this limit indicates a fault, while smaller exceedances are normal and do not require intervention. The minimum stress BOL, denoted here as 23, is used as the lower limit. The stress window bounded by these two curves is in the Fig. 4 is highlighted with hatching. The total voltage U is located at this point. iProvided that the voltage across the n = 2 cells in this range does not fall below the lower limit voltage, safe operation of the fuel cells is ensured, regardless of their aging. This establishes a generally small acceptable voltage range, which can lead to the erroneous detection of faulty fuel cells, since a voltage slightly below curve 23 would also be tolerable for aged cells.

[0023] The voltage of the individual fuel cells should be above U Limit This information must be kept in order to react in time and, if necessary, initiate countermeasures. Alternatively, to provide an additional safety buffer, U can be used instead. Limit a second lower limit voltage U Limit,2 used that are slightly higher than U Limit is: ULimit,2>ULimit

[0024] At this second lower limiting voltage U Limit,2Although no damage to the cell is expected yet, this indicates that the cell voltage has dropped significantly and that countermeasures should at least be prepared. This allows more reaction time for such countermeasures and enables more stable control of the fuel cell. When monitoring n cells, this second voltage limit applies accordingly. Umin,n,2=(n−1)⋅Uexp+ULimit,2

[0025] The tolerable voltage window becomes smaller the more cells are measured together by the cell voltage monitoring unit 17. If – as in the example above – two cells are measured in series, the voltage window is relatively large and the controller reliably indicates a malfunction of individual cells.

[0026] If the voltage across the n cells drops below U min,n If two lower voltage limits U are exceeded, the cell voltage monitoring unit 17 will issue an error message.Limit and U Limit,2 When used, upon reaching U min,n,2 A warning will be issued and only when U is reached min,n An error message or an alarm will be displayed. In this case, the stack can also be automatically shut down to prevent irreversible damage to the stack without requiring external intervention.

Claims

[1] Method for operating an electrochemical stack (10) comprising a plurality of electrochemical cells (1), each comprising an anode compartment (2) with an anode electrode (6) and a cathode compartment (3) with a cathode electrode (7), wherein the anode compartment (2) and the cathode compartment (3) are separated from each other by a semipermeable membrane (8) and an electrical voltage is present between the anode electrode (6) and the cathode electrode (7) during operation, wherein the electrochemical cells (1) are connected in series, and with a cell voltage monitoring system (17) connected to the electrochemical cells, characterized by - Measuring the electrical voltages U i using the cell voltage monitoring system (17) of at least n electrochemical cells connected in series, where n is greater than or equal to 2, - Comparing the measured voltages U i with an expected voltage Uexp,n and a minimum voltage U min,n , where the expected voltage U exp,n n times the expected cell voltage U exp is and U min,n (n-1) times the expected cell voltage plus a lower voltage limit U Limit , where the lower stress limit U Limit a minimum limit voltage up to which operation of a single electrochemical cell (1) is to be carried out, - Output an error message if at least one of the measured voltages U i smaller than U min or larger than U exp is. [2] Method according to claim 1, characterized by that the expected cell voltage U expof a single electrochemical cell (1) is determined from the voltage-current characteristic or the voltage-current density characteristic (UI characteristic) at the applied current or current density, where the current density is the current through the membrane of an electrochemical cell per unit area (A / m²). 2 ). [3] Method according to claim 2, characterized by , that the UI characteristic curve is determined by a measurement on one or more electrochemical cells (1). [4] Method according to claim 3, characterized by , that the UI characteristic curve is adapted to the lifetime of the electrochemical stack (10) by regularly determining the UI characteristic curve by a remeasurement. [5] Method according to any one of claims 1 to 4, characterized by , that the UI characteristic curve is determined using a model calculation, in particular taking into account the previous operating hours of the electrochemical stack (10). [6] Method according to claim 5, characterized by , that the UI characteristic curve is regularly recalculated during the operation of the electrochemical stack (10) by repeating the model calculation taking into account the previous operating hours or by taking an updated UI characteristic curve from a map stored in a control unit. [7] Method according to any one of claims 1 to 6, characterized by , that if the minimum voltage U is undershot min,n the electrochemical stack (10) is switched off. [8] Method according to any one of claims 1 to 7, characterized by , that a warning is issued if one of the measured electrical voltages U i smaller than a second minimum voltage U min,n,2 is, where U min,n,2 the sum of (n-1) times the expected voltage U exp a single cell (1) and a second lower voltage limit U Limit,2 is, where U Limit,2greater than the minimum limiting voltage U Limit and smaller than the expected cell voltage U exp is. [9] Method according to claim 8, characterized by , that if the second minimum voltage U is undershot min,n,2 the electrochemical stack (10) is switched off.

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