Method for arranging fuel cells in fuel cell stack in dependence on characteristics

Abstract

The invention relates to a method for the characteristic-dependent arrangement of fuel cells (12) within a fuel cell stack (10), comprising the following method steps: a) characterizing individual or several fuel cells (12) prior to the arrangement thereof in the fuel cell stack (10) with respect to their susceptibility to different fault situations (30, 40, 50, 60, 70); b) arranging the fuel cells (12) characterized according to method step a) within the fuel cell stack (10) in mounting positions (80, 82, 84, 86) in such a way that during operation of the fuel cell stack (10), the influence of the fault situations (30, 40, 50, 60, 70) ascertained according to method step a) on the fuel cells (12) is minimized.

Description

Technical Field

[0001] The present invention relates to a method for the property-dependent arrangement of fuel cells in a fuel cell stack. Prior to their arrangement in the fuel cell stack, the fuel cells or fuel cells are characterized with respect to specific cell properties, from which susceptibility to different fault scenarios is derived. Background Art

[0002] PEM fuel cells operating on hydrogen emit only water as exhaust gas and enable fast refilling times. Because individual fuel cells only provide a low voltage of 0.65 to 0.85 volts, in automotive applications, for example, hundreds of fuel cells are connected in series to generate a voltage level that is favorable for power electronics. For cost reasons, the individual fuel cells within a fuel cell stack are typically all identical components. Due to manufacturing tolerances, individual cells can differ in various parameters, such as the geometry of the flow field, the distribution of the catalyst, and the quality of the hydrophobic or hydrophilic coating. Furthermore, locations within the fuel cell stack may have better or worse flow through the fuel cells or may experience slightly higher or lower temperatures than other locations within the stack.

[0003] Currently, individual fuel cells are positioned within a fuel cell stack independently of their manufacturing tolerances. However, this means that fuel cells that are particularly susceptible to certain fault conditions, such as flooding, due to their manufacturing tolerances, may be positioned within the fuel cell stack at locations where the medium supply is particularly unfavorable for such fault conditions compared to other locations within the fuel cell stack. This results in reduced system efficiency and a shortened service life of the individual fuel cells and, potentially, the entire fuel cell stack.

[0004] On the other hand, if all fuel cells are to be operated reliably under all circumstances, regardless of their position within the fuel cell stack, it is necessary to ensure that manufacturing tolerances are reduced. However, this involves significantly increased costs for quality assurance, tools, and inspection methods during manufacturing. Furthermore, production management must be adjusted in this case to ensure that all fuel cells are adequately supplied even with large manufacturing tolerances. This requires significantly increased gas flows to all fuel cells to reliably prevent flooding of the most vulnerable cells. However, this means that optimal system-level operation is not possible and the costs of auxiliary units are significantly increased. Summary of the Invention

[0005] According to the present invention, a method for the property-dependent arrangement of fuel cells within a fuel cell stack is proposed, comprising the following method steps:

[0006] a) characterizing a single fuel cell or multiple fuel cells with respect to their susceptibility to different fault scenarios before they are arranged in a fuel cell stack;

[0007] b) Arranging the fuel cells of the fuel cell stack characterized according to method step a) in an installation level such that the influence of a fault event on the fuel cells ascertained according to method step a) during operation of the fuel cell stack is minimized.

[0008] The method proposed according to the present invention allows individual fuel cells to be sorted before assembly in a fuel cell stack, so that fuel cells with specific susceptibilities to failure are used and, in particular, are applied at locations within the fuel cell stack where, despite their susceptibility to failure, favorable operating conditions exist. This allows fuel cell stacks composed of identical fuel cells to be constructed in a manner that supports system efficiency. In this context, susceptibility to failure should also be understood as a certain loss in fuel cell efficiency, even if the fuel cell geometry remains within predefined tolerances. Consequently, a failure does not necessarily lead to fuel cell failure, but can simply result in a reduction in fuel cell performance. The method according to the present invention therefore improves the performance of a fuel cell stack by advantageously placing individual fuel cells or their components within the stack.

[0009] Advantageously, the method proposed according to the present invention allows the susceptibility of individual or multiple fuel cells to one or more fault scenarios to be determined. The detection of these fault scenarios advantageously reveals the installation position of the relevant fuel cells within the fuel cell stack. The susceptibility to fault scenarios can be derived from specific cell properties, such as geometrical dimensions, such as the channel depth of the fuel cell flow field.

[0010] In the method proposed according to the invention, during operation of the fuel cell stack, higher temperatures prevail in the area between the end plates than below the upper end plate and above the lower end plate. Due to the known temperature levels, malfunction-prone fuel cells that prefer specific temperature values ​​can be optimally positioned within the fuel cell stack.

[0011] In the method proposed according to the invention, the fuel cell is preferably used with an increased inlet pressure and an increased gas flow close to the lower end plate, which means that the medium is supplied to the fuel cell stack at this lower end plate.

[0012] The method proposed according to the present invention takes into account the identified characteristics of individual fuel cells or a plurality of fuel cells, in particular those resulting from individual manufacturing tolerances. The fuel cells or their components are positioned within the stack based on specific cell characteristics. These cell characteristics can be used to infer susceptibility to failure or an increased risk of a power reduction related to the installation location.

[0013] This includes, for example, a high susceptibility to the fault condition "flooding," which results in the second installation level of the fuel cells in question being located close to the lower end plate within the fuel cell stack. This means that if the specific cell characteristics of the fuel cells indicate an increased susceptibility to flooding, or a higher risk of performance degradation due to flooding, these fuel cells are placed close to the medium supply and discharge on the lower end plate, where the increased flow rate counteracts flooding of the fuel cells.

[0014] Furthermore, according to the method proposed in the present invention, fuel cells whose characteristic properties, due to individual manufacturing deviations, have a higher susceptibility to the fault condition "high-mass transmission loss" are arranged in the first mounting position layer within the fuel cell stack, in particular close to the upper end plate, according to the pressure distribution and gas flow distribution, in order to achieve a high outlet pressure.

[0015] In addition, the method proposed according to the present invention makes it possible to arrange fuel cells whose characteristic characteristics have a higher susceptibility to the fault condition "high-quality transmission loss" due to individual manufacturing deviations in a second installation layer close to the lower end plate within the fuel cell stack according to the pressure distribution and gas flow distribution to achieve a high gas flow.

[0016] Furthermore, the method proposed according to the invention makes it possible to arrange fuel cells whose characteristic properties, due to individual production deviations, are highly susceptible to the fault event "high-ohmic losses" in the first installation level within the fuel cell stack, close to the upper end plate.

[0017] Finally, by means of the method proposed according to the present invention, fuel cells whose characteristic properties, due to individual manufacturing deviations, have a high susceptibility to the fault situation "high activation loss" and / or to the fault situation "hazard of icing" are arranged in a second mounting position layer above the lower end plate within the fuel cell stack.

[0018] Therefore, the method according to the present invention allows normal fuel cells to be positioned arbitrarily within the fuel cell stack. The method according to the present invention can also be used after a specific operating period of the fuel cell stack has expired to reorder the fuel cells within the fuel cell stack. This allows not only a complete repetition of the method or its application on a reduced scale to the fuel cell stack, but also the reordering of aged fuel cells within the fuel cell stack.

[0019] In particular, in the method proposed according to the invention, individual, excessively aged fuel cells can be replaced by new fuel cells after a specific operating period of the fuel cell stack has expired.

[0020] Advantages of the invention

[0021] The method proposed according to the present invention allows individual fuel cells to be characterized with respect to their different susceptibilities to failure before being arranged in a fuel cell stack. In a second step, the individual, previously characterized fuel cells are arranged within the fuel cell stack so that their installation position is favorable with respect to susceptibility to failures caused by manufacturing deviations. This prevents individual cell failures that could lead to excessive aging of the cells involved and associated performance losses. Overall, this results in higher system efficiency and a longer service life for the individual fuel cells or fuel cell stacks. The method proposed according to the present invention, particularly the characterization of the properties of the individual fuel cells before the fuel cell stack is constructed, allows for greater tolerances to manufacturing tolerances, resulting in significant cost reductions in the production of common fuel cell components, as increased costs for quality assurance, tools, and inspection methods do not need to be maintained during the production process.

[0022] By means of the solution proposed according to the invention, it is possible to achieve an efficiency-optimized system operation by approaching the operating limits more closely and thus to increase the system efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The embodiments of the present invention will be further explained below with reference to the accompanying drawings and the following description.

[0024] The accompanying drawings show:

[0025] Figures 1.1 to 5.4 Individual fault conditions, tests for identifying the fault condition, possible causes and possibilities for avoiding the fault condition,

[0026] Figure 6 Schematic structure of a fuel cell stack,

[0027] Figure 7 、 8 , 9 according to Figure 6 Different installation-site-dependent parameters of the individual fuel cells within a fuel cell stack, such as temperature, pressure, and gas flow, are measured.

[0028] In the following description of embodiments of the present invention, identical or similar elements are identified with the same reference numerals, wherein a repeated description of these elements is omitted in individual cases. The figures merely schematically illustrate the subject matter of the present invention. DETAILED DESCRIPTION

[0029] Depend on Figures 1.1 to 5.4 The schematic diagram shows the tests for identifying a fault situation, the possible causes of the fault situation and the possibilities for avoiding the respectively occurring fault situation.

[0030] Figures 1.1 to 1.4 The "flooding" fault condition 30 is shown. A possible test for carrying out a test 32 for this "flooding" fault condition 30 is, for example, to reduce the gas velocity at different current intensities, wherein the cause 34 of the "flooding" fault condition 30 may be a defective coating, flow field, or geometry of the gas diffusion layer. This "flooding" fault condition 30 can be avoided 36 by an installation location of the relevant fuel cell 12 with a high gas flow rate.

[0031] Attachment Figures 2.1 to 2.4 The figure sequence shows the fault condition "loss of high-quality transmission" 40. A test 42 for the fault condition "loss of high-quality transmission" 40 is performed, for example, by evaluating the UI characteristic curve using electrical impedance spectroscopy (EIS). The cause 44 of the fault condition "loss of high-quality transmission" 40 lies, for example, in the geometry of the flow field or the gas diffusion layer and the distribution of the catalyst. The fault condition "loss of high-quality transmission" 40 can be avoided 46 by arranging the fuel cell 12 at the installation site at a location in the fuel cell 10 where high gas flows and increased outlet pressures are present.

[0032] Attachment Figures 3.1 to 3.4 The fault condition "ohmic loss" 50 is indicated. The test 52 for proving the fault condition "ohmic loss" 50 consists in evaluating the UI characteristic curve of the high-frequency resistance (HFR). The cause 54 for the fault condition "ohmic loss" 50 lies in the design of the membrane (CL = Catalyst Layer), in the design of the interface, or in the water transport properties of the gas diffusion layer. The occurrence of the fault condition "ohmic loss" 50 is avoided 56 by the installation location of the relevant fuel cell 12 within the fuel cell stack 10, where relatively low gas flows and low temperatures exist within the fuel cell stack 10.

[0033] Attachment Figures 4.1 to 4.4 The fault condition "high activation loss" 60 is indicated. The test 62 for the fault condition "high activation loss" 60 consists in evaluating the UI characteristic curve by means of electrical impedance spectroscopy (EIS). The cause 64 of the fault condition "high activation loss" 60 may be a reduced effective catalyst surface. The occurrence of the fault condition "high activation loss" 60 is avoided 66 by an installation location of the fuel cell 12 in question, which results in relatively high gas flows and high temperatures within the fuel cell stack (10).

[0034] Attachment Figures 5.1 to 5.4The fault condition "Icing Danger" 70 is indicated. The test 72 for the "Icing Danger" fault condition 70 includes humidification and drying cycles and a measurement of the current water content. The cause 74 of the "Icing Danger" fault condition 70 lies in the water transport properties of the membrane or gas diffusion layer, as well as in the flow field and gas diffusion layer geometry of the fuel cell 12. The effects of this fault can be avoided 76 or significantly reduced by installing the relevant fuel cell 12 within the fuel cell stack 10 at a location where relatively high gas flows and relatively high temperatures occur within the fuel cell stack 10.

[0035] An overview of the sequence of figures 1.1 to 5.4 shows: five different fault situations 30, 40, 50, 60, 70, which are demonstrated by tests 32, 42, 52, 62, 72, their possible causes 34, 44, 54, 64, 74 and the possibility of avoiding 36, 46, 56, 66, 76 them or reducing the effects of the fault situations 30, 40, 50, 60, 70 by a suitable installation position of the relevant fuel cell 12 within the fuel cell stack 10.

[0036] Since the method according to the present invention requires a single test of a fuel cell 12 or its components, cost-effective, easily executable test methods are preferred. Some conceivable methods for determining parameters related to the susceptibility of a fuel cell 12 during operation are briefly outlined below.

[0037] The flow field geometry can be measured using photographic methods, allowing the dimensional accuracy of individual channels in the flow field and the internal cell structure to be checked quickly and cost-effectively. To further reduce costs, a limited number of channels can be selected based on random criteria and tested for dimensional accuracy to draw conclusions about the geometric quality of the current flow field. Local narrowing or insufficient channel depth impairs the flow through the affected channels and increases the susceptibility of the corresponding fuel cell sample to the fault conditions "flooding" 30, "loss of high-quality transmission" 40, and "hazard of icing" 70, because in these cases, water was not optimally removed during the previously performed drying process.

[0038] The layer thickness can be measured during graphitization, that is, after the graphite has been applied to the stainless steel flow field, and the expected transition resistance can be determined during this measurement. This transition resistance contributes a constant fraction to the ohmic losses 50, which are additionally influenced by the membrane water loading during subsequent operation. If the fraction of the transition resistance is known, the membrane resistance fraction of the individual cells can be determined more accurately. Furthermore, with high contact resistances, higher heat generation can be expected in the fuel cell 12 in question. Therefore, placement at the edge of the fuel cell stack 10 is advantageous in order to homogenize the temperature profile across the fuel cell stack 10.

[0039] The ionomer distribution can be determined, for example, using a CO2 displacement measurement, by which the ionomer distribution in the catalyst layer can be determined. An unfavorable ionomer distribution changes the water transport properties and can affect the operation of the fuel cell stack 10 in various ways. Depending on the current distribution, the affected fuel cell 12 can dry out more frequently or flood more frequently than the remaining fuel cells 12. Depending on how the distribution deviates from the ideal state, different locations within the fuel cell stack 10 may be advantageous (see also the fault scenarios "Flooding" 30 or "High-ohmic Losses" 50).

[0040] The platinum loading of the electrodes can be determined, for example, by means of X-ray fluorescence measurements, from which conclusions can be drawn about the expected performance of the fuel cell 12. If the platinum loading is above average, a favorable cell behavior results, with relatively high voltage levels and a low risk of high mass transfer losses 40. This type of fuel cell 12 is therefore less sensitive to insufficient reactant supply. If, on the other hand, the platinum loading is below average, the voltage level of the fuel cell 12 deteriorates over the entire operating range. In particular, the mass transfer losses 40 can increase significantly due to the reduction in active catalyst surface. If this type of fuel cell 12 is identified, it should be placed at a location within the fuel cell stack 10 that provides a better-than-average reactant supply in order to avoid a strong voltage drop in the fuel cell 12 in question during subsequent full-load operation.

[0041] As an alternative to X-ray methods, a layer thickness measurement of the counter electrode can also be performed. If the electrode is thinner than average, this can also indicate a below-average platinum loading.

[0042] If the dispersion of manufacturing parameters is controllable, for example within a batch, and deviations between individual batches occur, particularly due to tool changes, temperature variations, or raw material variations, individual checks of individual fuel cells 12 can be dispensed with completely. By sampling within a batch, a manufacturing parameter deviation can be assigned to the batch with a certain statistical probability. According to the present invention, individual fuel cells 12 from a batch in which, for example, channel narrowing is observed due to sampling can be advantageously distributed across multiple fuel cell stacks 10. Cell locations within the fuel cell stack 10 that are particularly critical for flooding are then occupied by cells from batches without anomalies.

[0043] In order to characterize the determined properties of the fuel cell 12 with still unknown production deviations, various tests are performed, as described in the accompanying Figure 1.2 、 2.2 , 3.2, 4.2, 5.2. For example, in order to evaluate the susceptibility of the fuel cell 12 to the fault condition "flooding" 30, the flow rate at the anode or cathode is gradually reduced. If flooding 30 occurs abnormally early in this sample of fuel cells 12, this is most likely due to deviations in the coating quality or geometric deviations in the flow field or the gas diffusion layer. In order to reduce the performance degradation caused by this fault condition or its effects during the operation of the fuel cell stack 10, the fuel cell 12 in question should therefore be placed at a location within the fuel cell stack 10 where the gas flow rate is higher than at other locations within the fuel cell stack 12. The flooding of the fuel cell 12 is therefore offset by the relatively high gas flow rate; the risk of performance degradation due to flooding is minimized.

[0044] For the remaining fault cases “high-quality transmission losses” 40 , “high-ohmic losses” 50 , “high activation losses” 60 and “risk of icing” 70 , requirements regarding the relative position at which the tested fuel cell 12 is arranged within the fuel cell stack 10 are made according to the same method.

[0045] Figure 6 A fuel cell stack 10 is shown, which consists of a plurality of fuel cells 12 arranged vertically one above the other. The fuel cell stack 10 has an upper end plate 14 and a lower end plate 16. An inflowing cooling medium 18 enters the fuel cell stack 10 and leaves the stack again as outflowing cooling medium 20. The inflowing hydrogen is indicated by position 22, and the outflowing hydrogen is indicated by position 24. The inflowing air 26 leaves the fuel cell stack 10 again as outflowing air 28. Different gas flows or mass flows are arranged according to the Figure 6 In the schematic diagram, they are marked by arrows in the corresponding directions.

[0046] exist Figure 6In the illustrated fuel cell stack 10, the temperature in the center of the fuel cell stack 10, equidistant from the upper end plate 14 and the lower end plate 16, tends to be higher than the temperature in the region of the upper or lower end plates 14, 16. Simultaneously, the fuel cells 12 arranged near the lower end plate 16 operate at slightly higher inlet pressures and gas flows. Due to their individual manufacturing tolerances, the fuel cells 12 are susceptible to the "flooding 30" fault and should therefore be placed near the lower end plate 16, and therefore in the second mounting level 82 in the fuel cell stack 10.

[0047] Fuel cells 12 that are more susceptible to the "mass transmission loss" fault condition 40 require higher gas flow rates and / or higher outlet pressures to achieve higher reactant partial pressures. Depending on the pressure and gas flow distribution, a mounting level close to the lower end plate 16, i.e., the second mounting level 82, or a mounting level in the region of the upper end plate 14, i.e., the first mounting level 80, can be selected for this purpose. In this particular case, the mounting level depends on the specific design.

[0048] Fuel cells 12 that are highly susceptible to the "high ohmic losses" fault condition 50 should, in contrast, be humidified above average. For this purpose, the first mounting level 80 in the region of the upper end plate 14 is suitable because relatively low temperatures and low gas flows prevail there. The risk of performance degradation of the fuel cells 12 due to ohmic losses is minimized for fuel cells 12 within the first mounting level 80.

[0049] Tests have shown that fuel cells 12 that are susceptible to the fault condition "high activation loss" 60 or the fault condition "hazard of icing" 70 should be positioned toward the lower end plate 16, that is, in the lower region of the fuel cell stack 10, but not directly on the lower end plate 16, as high temperatures and gas flows occur there. For such fuel cells 12, the third or fourth mounting level 84, 86 within the fuel cell stack 10 is suitable.

[0050] Fuel cells 12 that do not show any particular susceptibility to the listed fault situations 30 , 40 , 50 , 60 , 70 can be distributed arbitrarily at still free positions, i.e. they can be arranged centrally, for example in the region of the third mounting level 84 , the fourth mounting level 86 or the fifth mounting level 88 within the fuel cell stack 10 .

[0051] from Figure 7A temperature profile 104 of temperature 102 is shown, plotted over position 100 of an individual fuel cell 12 within fuel cell stack 10. A substantially uniform temperature prevails in the center of fuel cell stack 10, while a significantly lower temperature range 106 exists in the region of upper end plate 14 and lower end plate 16.

[0052] Figure 8 The curve of the pressure 108 is shown, which is also plotted with respect to the position 100 of the relevant fuel cell 12 within the fuel cell stack 10. Figure 8 The diagram shows that the inlet pressure curve 110 has a negative gradient starting from the lower end plate 16 to the upper end plate 14, which is caused by the pressure loss. The outlet pressure curve 112 has a more positive gradient, but at a lower pressure level.

[0053] Finally from Figure 9 A gas flow 116 can be seen, which is also plotted relative to the position 100 of the fuel cell 12 within the fuel cell stack 10. The gradient 118 has a negative slope here.

[0054] Because the individual fuel cells 12 arranged within the fuel cell stack 10 age unevenly, it is useful to change the order of the fuel cells 12 within the fuel cell stack 10 over the operating time. To this end, the method proposed according to the present invention is repeated completely or on a reduced scale during the service life, and the order of the individual fuel cells 12 is optionally re-ordered. Furthermore, it is also possible to replace individual excessively aged fuel cells 12 with new fuel cells 12 during this step.

[0055] The invention is not limited to the embodiments described herein and the aspects highlighted therein. Instead, numerous modifications within the scope of the person skilled in the art are possible within the scope of the claims.

Claims

1. A method for arranging fuel cells (12) in a fuel cell stack (10) in a property-dependent manner, comprising the following method steps: a) characterizing a single fuel cell or multiple fuel cells (12) with respect to their susceptibility to different fault scenarios (30, 40, 50, 60, 70) before they are arranged in a fuel cell stack (10); b) arranging the fuel cell (12) characterized according to method step a) in the fuel cell stack (10) in an installation level (80, 82, 84, 86) such that the influence of the fault condition (30, 40, 50, 60, 70) determined according to method step a) on the fuel cell (12) is minimized during operation of the fuel cell stack (10).

2. The method according to claim 1, characterized in that The susceptibility of one or more fuel cells (12) to the occurrence of one or more fault conditions (30, 40, 50, 60, 70) determined according to method step a) results in an installation level (80, 82, 84, 86) of the one or more fuel cells in the fuel cell stack (10).

3. The method according to claim 1 or 2, characterized in that During operation of the fuel cell stack (10), a higher temperature prevails in the region between the end plates (14, 16), while a lower temperature (106) prevails below the upper end plate (14) and above the lower end plate (16).

4. The method according to claims 1 to 3, characterized in that The fuel cell (12) adjacent the lower end plate (16) operates at an increased inlet pressure (110) and an increased gas flow (116).

5. The method according to claims 1 to 4, characterized in that A fuel cell (12) whose characteristic properties, due to individual manufacturing deviations, have a higher susceptibility to the fault condition "flooding" (30) is arranged in the fuel cell stack (10) in a second installation level (82) close to the lower end plate (16).

6. The method according to claims 1 to 4, characterized in that Fuel cells (12) whose characteristic property is a high susceptibility to the fault condition "mass transmission loss" (40) due to individual manufacturing tolerances are arranged in the first installation level (80) of the fuel cell stack (10) close to the upper end plate (14) according to the pressure distribution and gas flow distribution to achieve a high outlet pressure.

7. The method according to claims 1 to 4, characterized in that The fuel cell (12), whose characteristic property is a high susceptibility to the fault condition "mass transmission loss" (40) due to individual manufacturing deviations, is arranged in the second installation level (82) of the fuel cell stack (10) close to the lower end plate (16) according to the pressure distribution and gas flow distribution to achieve a high outlet pressure.

8. The method according to claims 1 to 4, characterized in that A fuel cell (12) whose characteristic properties, due to individual manufacturing deviations, have a higher susceptibility to the fault condition "high ohmic losses" (50) is arranged in the first installation level (80) of the fuel cell stack (10) near the upper end plate (14).

9. The method according to claims 1 to 4, characterized in that Fuel cells (12) whose characteristic properties, due to individual manufacturing deviations, have a higher susceptibility to the fault condition "high activation loss" (60) and / or to the fault condition "hazard of icing" (70) are arranged in a second mounting level (82) above the lower end plate (16) in the fuel cell stack (10).

10. The method according to claims 1 to 4, characterized in that Fuel cells (12) without abnormalities are randomly placed within the fuel cell stack (10).

11. The method according to claims 1 to 10, characterized in that The fuel cells (12) are reordered after the operating period of the fuel cell stack (10) has ended.

12. The method according to claim 11, characterized in that The method according to claims 1 to 10 is repeated in its entirety or on a reduced scale, or is carried out within the scope of a reordering of the fuel cells (12) within the fuel cell stack (10).

13. The method according to claim 11, characterized in that Within the fuel cell stack (10), individual excessively aged fuel cells (12) are replaced with new fuel cells (12).

Images