WO2021121778A1

WO2021121778A1 - Cell stack having at least one clamping device

Info

Publication number: WO2021121778A1
Application number: PCT/EP2020/081500
Authority: WO
Inventors: Jochen Wessner; Harald Bauer
Original assignee: Robert Bosch Gmbh
Priority date: 2019-12-17
Filing date: 2020-11-09
Publication date: 2021-06-24
Also published as: DE102019219782A1

Abstract

The present invention relates to a cell stack (1), in particular a fuel cell stack. The cell stack (1) comprises several individual cells (4) which are clamped between two end plates (2, 3) by means of at least one clamping device (10). The at least one clamping device (10) consists of a carbon fiber composite material.

Description

description

Title:

Cell stack with at least one clamping device

The present invention relates to a cell stack with at least one clamping device.

State of the art

In fuel cell systems, the oxidizing agent oxygen from the ambient air is used as a rule in order to react in the fuel cell - in the individual cells - with hydrogen to form water and thus to deliver electrical power through electrochemical conversion.

From US5789091 A a fuel cell stack is known in which a large number of individual cells are clamped between two end plates by means of tensioning straps.

For the functions of “electrical conductivity” and “tightness” in the fuel cell stack, the individual cells must be pressed against each other, which is done with the help of the tensioning straps. In order to keep the tensioning force required for this as constant as possible even over changes in length of the fuel cell stack, spring assemblies are known from EP1601041B1, for example, which are attached between an end plate and a further plate.

Disclosure of the invention In contrast, the cell stack according to the invention has changes in length of the cell stack which are comparatively easy to control and which are caused, for example, by thermal stress or by swelling of membranes in the individual cells.

For this purpose, the cell stack comprises a plurality of individual cells, the individual cells being clamped between two end plates by means of at least one clamping device. The at least one tensioning device consists of a tensioning material, the tensioning material having a coefficient of thermal expansion less than 2.5 * 10 ^L -6 m / (m * K), preferably less than 1.0 * 10 ^L -6 m / (m * K) ), owns. Advantageously, carbon fiber composite material is used as the tensioning material; Another advantageous clamping material is Invar, an iron-nickel alloy with the material number 1.3912. The specified coefficient of thermal expansion should apply to room temperature.

As a result, the clamping device has a very low coefficient of thermal expansion compared to the individual cells; In the individual cells there can be components made of steel, for example, which have a coefficient of thermal expansion of approx. 11.5 * 10 ^L -6 m / (m * K). Thus, the thermal expansion of the tensioning device compared to the thermal expansion of the individual cells can be virtually neglected when designing the cell stack and also when regulating the tensioning force during operation of the cell stack.

The tensioning material advantageously has a modulus of elasticity greater than 90,000 N / mm ² . The modulus of elasticity applies to the direction in which the force is transmitted by the clamping device. The tensioning material is thus stiffer and preferably also stronger than, for example, aluminum. The clamping material is therefore well suited for the function of the clamping device. The tensioning material made of carbon fiber composite material, which has a modulus of elasticity of> 130,000 N / mm ² in the direction of the fibers, is particularly preferred; the fiber direction is then logically arranged in the direction of the force transmission of the tensioning device. The cell stack preferably has an adjusting element. This can be a piezo actuator, for example. The tension acting on the individual cells is applied there as a surface pressure in the individual elements. The adjusting element or the resulting tension that is as constant as possible ensures that the functions of "electrical conductivity" and "tightness" of the cell stack are fulfilled without impairing the service life of the cell stack, which is the case with excessive surface pressure in the individual cells - and there in particular in the membranes - would be the case. Mechanical re-tensioning of the tensioning device during maintenance of the cell stack can also be dispensed with.

The adjusting element can act, for example, directly on the clamping device or on one of the two end plates. The actuating element can be designed so that the individual cells can be loaded and / or relieved. The design depends in particular on whether the tension is to be adapted primarily with regard to a setting behavior of the cell stack or a swelling of the membranes of the individual cells or a temperature load on the cell stack. The thermal expansion of the tensioning material can now be used for the corresponding calculations.

In advantageous developments, the adjusting element has a spacer. The spacer can act both inwardly towards the individual cells and outwardly towards a housing, so that, for example, target distances are maintained even with horizontal assembly or horizontal installation; in this way, for example, electrical short circuits or excessive bending of the cell stack can be prevented.

The adjusting element is preferably integrated in the clamping device or in the cell stack. In this way, the tensile forces prevailing in the clamping device can be adjusted very well.

In this case, the actuating element particularly preferably has a greater heat transfer coefficient than the individual cells. Thus, in the event of a temperature increase during operation of the cell stack, the low thermal expansion of the clamping device due to the high thermal expansion of the actuating element are compensated, so that overall an average thermal expansion, comparable to that of the individual cells, is achieved.

In advantageous embodiments, the tensioning device is designed as a tensioning strap. The tensioning strap is very resilient in relation to its material cross-section in order to ensure the necessary tensioning force for the fuel cell stack. Alternatively, however, the clamping device can also be designed as a tie rod or as a housing.

In advantageous developments, the tensioning band has a reduced cross section in an expansion area. Due to the reduced cross-section in the expansion area, sufficient travel is now available even under load, i.e. the tensioning strap can stretch so that the length of the fuel cell stack - for example due to the swelling of the membranes of the individual cells - can be absorbed without major changes in the tensioning force. As a result, the individual cells are neither compressed too much, so that, for example, perforations in the membranes are avoided, nor are the functions of “electrical conductivity” and “tightness” impaired. At the same time, the tension band retains its strength at its attachment points to the end plate, since the cross section is not reduced here. The tension band cannot be weakened in the area of the fastening points, since otherwise the force application zones - or, for example, the areas for welded connections - would be weakened. A thinning of the entire tightening strap should therefore be deliberately avoided. The load-bearing capacity of the tensioning band in the area of the introduction of force at the fastening points is thus retained.

In an advantageous embodiment, the reduced cross section is achieved by means of a waist. The waist refers to the width of the strap.

In an alternative embodiment, the reduced cross section is achieved by means of a thinning. The dilution relates to the thickness of the tensioning strap. In preferred developments, the individual cells each have a membrane with a maximum thickness of 50 μm - particularly preferably a maximum of 20 μm - and gas diffusion layers with a maximum thickness of 150 μm - particularly preferably a maximum of 100 μm. Each individual cell usually has two gas diffusion layers. For this comparatively thin design of the individual cells, it is particularly advantageous to absorb the changes in length of the fuel cell stack via cross-sectional tapering of the tensioning straps without having to use additional aids such as spring assemblies, since the absolute changes in length of the fuel cell stack are comparatively small for this design.

The invention additionally comprises a motor vehicle with the cell stack according to the invention designed as a fuel cell stack. The individual cells are therefore designed as fuel cells. The motor vehicle has the same advantages mentioned for the cell stack. A fuel cell stack that does not require any maintenance intervals to readjust the tensioning of the clamping device is of great advantage, particularly for mobile applications.

Exemplary embodiments of the invention are shown in the drawings and explained in more detail in the description below. It shows:

FIG. 1 schematically shows a cell stack known from the prior art.

FIG. 2 shows a cell stack according to the invention designed as a fuel cell stack in a perspective view, only the essential areas being shown.

FIG. 3 shows a further cell stack in a section, only the essential areas being shown.

FIG. 4 shows yet another cell stack in a section, only the essential areas being shown. FIG. 5 shows a section through a cell stack with an adjusting element, only the essential areas being shown.

FIG. 1 shows schematically a cell stack 1 designed as a fuel cell stack, as is known from US5789091A. The fuel cell stack 1 has a plurality of individual cells 4 designed as fuel cells, which are clamped between two end plates 2, 3. The tensioning takes place by means of two tensioning devices 10 designed as tensioning straps, which extend tightly around the end plates 2, 3 and the individual cells 4, so that the fuel cell stack 1 is held and secured in its assembled state.

In order to maintain a pretensioning force on the individual cells 4, spring assemblies between one of the end plates 2, 3 and a further plate are known, for example, from EP1601041B1. As a result, a contact force that is as uniform as possible is to be applied to the individual cells 4 and fluctuations in the contact force - for example as a result of thermal expansions - are to be reduced.

According to the invention, the tensioning devices 10 are now made of a tensioning material with a low heat transfer coefficient, preferably made of carbon fiber composite material. In this case, the clamping devices 10 can preferably be designed both as clamping straps and as tie rods, or even be designed in the form of a housing.

FIG. 2 schematically shows a cell stack 1 designed as a fuel cell stack with a plurality of individual cells 4 designed as fuel cells, which are clamped between two end plates 2, 3 by means of clamping devices 10 designed as clamping bands, in a perspective view. The individual cells 4 extend in the stacking direction, that is to say in the z direction, over a total height 4a.

In this embodiment, the tensioning straps 10 are fastened to the upper end plate 2 by means of two fastening points 6 each. In alternative In embodiments, each tension band 10 can of course also be attached to the upper end plate 2 and the lower end plate 3; the deflection via one of the end plates 2, 3 would then be omitted, but twice as many tensioning straps 10 and fastening points 6 would be required per fuel cell stack 1.

Alternatively, the clamping devices 10 can also be designed as tie rods or as a housing. The housing is arranged surrounding the cell stack 1 and can absorb tensile forces analogously to the tensioning straps and tie rods. The respective clamping device 10 consists of a carbon fiber composite material. FIG. 2 shows the tensioning devices 10 designed as tensioning straps in further-developing versions.

The middle tensioning strap 10.1 has a constant cross section, for example. The right tensioning band 10.2 has a reduced cross-section in the form of a waist over the expansion area 10a - that is, a cross-sectional tapering in its width B in the x-direction. The left tensioning band 10.3 has a reduced cross-section over its expansion area 10a in the form of a thinning of the band - that is, a cross-sectional tapering in its thickness D in the y-direction.

In a particularly preferred embodiment, the right tensioning strap 10.2 is tapered in such a way that with the same material thickness - i.e. constant thickness D, for example 1 mm - the tensioning strap 10.2 is tapered from a width B of 20 mm to 10 mm in the expansion area 10a. The transitions of the taper are advantageously shaped in a load-optimized manner, that is, they are rounded.

In a particularly preferred embodiment, the left tensioning strap 10.3 is thinned in such a way that the material thickness - that is, its thickness D - varies. With a constant width B of 20 mm, the tensioning band 10.3 advantageously has a thickness of, for example, 1.0 mm in the area of its fastening points 6, while it has a tapered thickness D of only 0.5 mm in the expansion area 10a. The transition from 1.0 mm to 0.5 mm can be produced particularly easily by pressing or forging.

If a roller is used for this, the production results in a rounded and thus load-optimized transition of the material thicknesses. The expansion area 10a of the reduced cross-section or the tapering preferably extends in its length L over the total height 4a of all individual cells 4, i.e. has almost the same length L as the height of an individual cell 4 multiplied by the number of individual cells 4 of the cell stack 1.

As a result, tensioning straps 10 are used which have a smaller cross-section in a direction perpendicular to the greatest extent of the strap, that is to say transverse to the main force direction z. This reduced cross-section lies in the lateral area of the fuel cell stack 1, i.e. along the z direction, preferably in the area of all stacked individual cells 4. This results in a relatively large expansion of the tensioning straps 10 in the middle area of the cell stack 1 - i.e. in the expansion area 10a - but flat no local elongation that is too great - with the ability to "softly" absorb load fluctuations during operation and / or geometrical fluctuations during assembly or differences in setting behavior; at the same time, a comparatively high load-bearing capacity of the tensioning straps 10 in the area of the introduction of force at the fastening points 6 is retained.

Additional spring assemblies, as known from the prior art, can thereby be dispensed with in a preferred embodiment; the entire cell stack 1 can be built more compact, lighter and cheaper.

In order to be able to ensure their function, the individual cells 4 are pressed against one another. As a result, on the one hand, the electrical contact resistance between the elements is minimized and, on the other hand, the inserted sealing elements are sufficiently compressed so that the cell stack 1 is gas-tight. For this purpose, the dimensionally stable end plates 2, 3 at the respective end of the cell stack 1 are preferably pressed evenly against one another.

Elements of the individual cells 4 can be elastic, such as, for example, a polymer membrane of a membrane-electrode arrangement, a gas diffusion layer or a bipolar plate. The membrane can, for example, experience an increase in thickness due to swelling behavior due to the accumulation of water. In order to maintain sufficient compression for the functions in spite of this swelling behavior, or in spite of the usual settling behavior hold, sufficiently elastic elements are introduced into the power chain by the tensioning straps 10 made of the tensioning material or carbon fiber composite material with cross-sectional tapering, so that an additional spring package can be omitted.

In particular, the tensioning straps 10 made of carbon fiber composite material with cross-sectional tapering meet the requirements even without the use of spring assemblies, if the individual cells 4 each have a membrane with a thickness (in the z-direction) of a maximum of 50 μm - particularly preferably a maximum of 20 μm - and two gas diffusion layers each each have a maximum thickness of 150 μm - particularly preferably a maximum of 100 μm. Then the length changes of the individual cells 4 to be compensated by the tensioning straps 10 are smaller than would be the case for thicker membranes and gas diffusion layers, in which more space would then already be available within each individual cell 4, in particular for locally varying swellings and compressions.

FIG. 3 shows a further cell stack 1 according to the invention in cross section in the area of a clamping device 10, only the essential areas being shown. In the embodiment of FIG. 3, the tensioning device 10 is designed as a tensioning strap, but can alternatively also be designed as a housing or tie rod. The design as a housing can even have the same cross-section.

This cross section shows how the tensioning band 10 is fastened to the lower end plate 3 at two fastening points 6. For this purpose, the tensioning band 10 forms a tab 6a each in the area of an attachment point 6, which is arranged around a bolt, which in turn is firmly connected to the end plate 3 or is made in one piece with it or is mounted on it, for example by means of a pin .

In the embodiment of FIG. 3, the tensioning strap 10 is designed in three parts, with three partial straps 10b, 10c, 10d. Two sub-belts 10b, 10c, 10d are connected by means of an adjusting element 11; the adjusting element 11 is therefore integrated into the clamping device 10. The adjusting element 11 is designed in further developments so that it is in the force flow of the tensioning band 10 reduced cross-section - or, equivalently, a reduced modulus of elasticity or a material with a flatter flow curve. The length of the expansion region 10a then preferably results from the length of the adjusting element 11.

The adjusting element 11 has an adjusting device with which the pretensioning of the partial bands 10b, 10c, 10d and thus the pretensioning of the tensioning band 10 can be adjusted. Furthermore, the adjusting element 11 preferably has a spacer 11a so that, for example, the cell stack 1 does not hit a housing wall when it is mounted horizontally, or so that a defined distance between the tensioning band 10 and the individual cells 4 can be ensured.

The design of the actuating element 11 can now be carried out more easily with the tensioning device 10 from the tensioning material, since the thermal expansion coefficient of the tensioning material is comparatively low and thus one less unknown is present in the series connection of different thermal expansion coefficients of the cell stack 1. If the cell stack 1 is heated, it would want to expand in the z-direction, among other things. If the clamping device 10 does not expand at the same time as the cell stack 1, the surface pressures in the individual cells 4 would increase. In order to counteract this effect, the adjusting element 11 now ensures that the clamping device 10 also expands and / or that the cell stack 1 shortens in the z direction.

FIG. 4 schematically shows a further cell stack 1 designed as a fuel cell stack in a section, only the essential areas being shown. As is known from the prior art, the cell stack 1 has a plurality of individual cells 4 which are clamped between the first end plate 2 and the second end plate 3 by means of the at least one clamping device 10. The individual cells 4 include - also known from the prior art - a membrane-electrode unit which is arranged between two bipolar plates, each bipolar plate almost half belonging to one of the two adjacent individual cells 4. Usually they each point to the End plates 2, 3 bordering individual cells 4 then a so-called monopolar plate.

In the embodiment of FIG. 4, the actuating element 11 is designed as a piezo actuator and is integrated into the cell stack 1, more precisely integrated into a cavity 15 of the cell stack. Alternatives are hydraulic, pneumatic and electromagnetic actuating elements 11. Preferably, the thermally induced changes in a length of the cell stack 1 in its z-direction during operation are compensated for by the actuating element 11, since the clamping device 10, which is preferably designed as a carbon fiber composite material, has a comparatively low coefficient of thermal expansion and is therefore less than Thermal stress does not carry out any thermal expansion. Thus, for the design of the cell stack 1, an unknown - namely the change in length of the clamping device 10 due to thermal stress - is eliminated; this is an unknown, because during the operation of the cell stack 1 the temperature distribution in the cell stack 1, in particular in the clamping device 10, cannot be determined precisely, in particular not during the starting process of the cell stack 1.

Furthermore, changes in length of the cell stack 1 are also caused by other physical effects, for example by swelling of moisture, and the cell stack 1 is also set plastically over time. Changes in length of the cell stack 1 caused in this way can thus also be compensated for by the adjusting element 11, so that ideally the tension forces acting on the individual cells 4 remain constant over the service life of the cell stack 1.

When the cell stack 1 is in operation, the adjusting element 11 can regulate the tensioning force acting on the individual cells 4 and thus also achieve optimal compression or surface pressure in the individual cells 4; the change in length of the clamping device 10 caused by the temperature load can be neglected. If, for example, a membrane of the individual cell 4 swells during operation due to changed humidity at a constant temperature, the mechanical force can be reduced via the adjusting element 11 in order to avoid excessive surface pressures and thus degradation of the individual cells 4. In preferred developments, the actuating element 11, especially if it is designed as a piezo actuator, is arranged in the cavity 15 of the cell stack 1, in particular in a cavity 15 designed as a fluid channel. As a result, the cell stack 1 remains compact. In the embodiment of FIG. 4, two cavities designed as fluid channels are shown. The fluid channels can conduct ambient air, hydrogen or cooling medium, for example. In the embodiment of FIG. 4, the fluid is passed through an inlet 8 through the lower end plate 3 into the right-hand cavity 15; from there the individual cells 4 are supplied with the fluid. From the individual cells 4, the fluid flows - consumed, unused or also reacts - further into the left cavity 15, and from there out of the cell stack 1 through an outlet 9, which also leads through the lower end plate 3.

FIG. 5 shows a section through a cell stack 1 with an actuating element 11 designed as a piezo actuator, only the essential areas being shown. The adjusting element 11 has a piezo element 23 which is arranged in a pot 22. The pot 22 is fixed to a sleeve 21 or is connected to it in one piece. The pot 22 preferably has a significantly shorter length in the z direction than the sleeve 21, which extends from the first end plate 2 to the second end plate 3. The sleeve 21 is fixed to the second end plate 3, in the embodiment of FIG. 5 by means of a press fit 21a. The sleeve 21 is not fixed on the first end plate 2, so that the sleeve 21 and pot 22 can move relative to the first end plate 2 at least in the z-direction.

The piezo element 23 is supported on a bottom of the pot 22 and acts on an active element 25, which is designed as a lever arm in the embodiment of FIG first end plate 2 can act. The actuating element 11 thus represents an operative connection between the first end plate 2 and the second end plate 3 in the embodiment of FIG Individual cells 4, which are clamped between the end plates 2, 3 are, and thus supports the effect of the tensioning device 10 or relieves the tensioning device 10.

In alternative embodiments, however, the adjusting element 11 can also be designed in such a way that it counteracts the effect of the clamping device 10 or loads the clamping device 10.

The cross section of the piezo actuator is comparatively small, so that it can be pushed into a cavity 15 of the cell stack 1 without any problems, especially if its end is inserted into a sleeve 21 that can transmit the force out of the cavity 15. In the area of the comparatively small piezo element 23, large forces are available spatially close to one another in the vicinity of an end plate 2, 3.

The cavity 15 is preferably a fluid channel of the cell stack 1. If the cell stack 1 is designed as a fuel cell stack, it has fluid channels for oxygen or air and for hydrogen, and in further versions also for cooling medium. In particularly preferred embodiments, the adjusting element 11 is then arranged in a fluid channel for cooling medium, so that the adjusting element 11 is well cooled.

The adjusting element 11 is particularly preferably not arranged on the end plate 3 at the inlet 8 of the fluid channels, but on the opposite end plate 2. Already halfway through a fluid channel, only half of the cross section is required for the fluid flowing through, since half of it is already required the individual cells 4 were supplied with fluid accordingly. In this way, for example, in the last third of a 300 mm high cell stack 1, a piezo actuator 100 mm long, which requires half the cross section of a fluid channel, can be inserted without disturbing the operation of the cell stack 1.

If the actuating element 11 designed as a piezo actuator is intended not only to build up or adjust mechanical tension, but also to enable a greater path, its longitudinal extension is preferably translated by means of the active element 25 that can be tilted about the axis of rotation 24. The active element 25 acts as a lever arm in the embodiment of FIG. 5 on the upper end plate 2 and ensures additional tensioning of the individual cells 4 when the piezo actuator expands, thus having a load on the individual cells 4 when energized and thus relieving the tensioning device 10. Such an effective direction can be used, for example, to compensate for the setting behavior of the cell stack 1 in order to maintain the surface pressure required for the functionalities. To compensate for the swelling of the membranes during operation of the cell stack 1, the individual cells 4 would then have to be relieved by the actuating element 11, for example by reducing the current flow for the structure according to FIG. 5 or by an opposing effective direction of the active element 25 on the upper end plate 2 .

The electrical voltage that is required to operate the piezo actuator can be tapped from a system of the cell stack 1, for example from a battery of a fuel cell system in which the cell stack 1 is arranged as a fuel cell stack.

In further embodiments, the cell stack 1 has one or more tension sensors which are suitable for determining or calculating the surface pressure in the individual cells 4. The tension sensor can be designed, for example, as a sensor film or as a piezo sensor. As a function of the surface pressure determined by the tension sensor in the individual cells 4, the adjusting element 11 can now be activated in order to set the optimum surface pressure. The adjusting element 11 can force both a shortening and an elongation of the cell stack 1.

As an alternative or in addition to this, the cell stack 1 can also have a moisture detector. The moisture detector detects swelling of the individual cells 4 or the membranes of the individual cells 4 due to water. The moisture detector can be a water detector, for example, or it can also be stored in a control unit of the cell stack 1 via a characteristic map. The actuating element 11 can then be controlled as a function of the humidity of the individual cells 4 or as a function of swelling in such a way that the surface pressure in the individual cells 4 remains essentially constant or in a defined range. The functioning of the cell stack 1 according to the invention is as follows:

A single cell 4 designed as a fuel cell usually comprises a bipolar plate and a membrane-electrode arrangement, more precisely two bipolar plate halves and a membrane-electrode arrangement. Furthermore, an individual cell 4 designed as a fuel cell comprises a plurality of seals. By stacking the metallic bipolar plates, membrane-electrode arrangements and seals, as well as other structures - such as graphitic gas diffusion layers - a spatially different series connection of materials and their coefficients of thermal expansion is created in the stacking direction. In addition, for example, polymeric membranes of the membrane-electrode arrangements can be swollen and cold or swollen and warm in the moist state. The cell stack 1 can also be still warm and the membranes can be drier or vice versa - such as when starting up, for example. Due to heat and sources, the cell stack 1 would experience corresponding changes in length in the stacking direction in the unpressed state.

The pressing of the cell stack 1 requires the application of a defined pressing for its robust function. As a rule, this is only possible to a limited extent in the prior art, since the tensioning device 10 of the cell stack 1 is also subject to thermal expansion. Although this can theoretically be taken into account computationally, in practice the restriction arises that the actual local temperature, in particular when starting up the cell stack 1 or also in transient operation, can only be predicted inadequately.

In the prior art, the cell stack 1 not only elongates in an undefined manner, the clamping device 10 also does not maintain a constant distance. In the prior art, attempts are made to compensate for this in part by means of compensating or expansion elements as well as active elements (for example pneumatic tension) or passive elements (such as spring assemblies).

In the case of the cell stack 1 according to the invention, however, the tension is set as precisely as possible via the adjusting element 11, depending on the design, passively or active. The cell stack 1 is clamped mechanically by the tensioning device 10, which can be, for example, a housing, several screws or tensioning straps. In this case, the axial force in the z-direction for clamping the two end plates 2, 3 of the cell stack 1 is transmitted, for example, through the carbon fiber structures of the clamping device 10. Since the coefficient of thermal expansion of steel - which is the typical material for the bipolar plates of the individual cells 4 - is -11.5 * 10 ^L -6 m / (m * K) compared to a carbon fiber composite material with -0.2 * 10 ^L -6 m / (m * K) is comparatively large, the cell stack 1 can be clamped in a much more defined manner when using the clamping device 10 made of carbon fiber composite material - or from a comparable clamping material - because the local expansion of the clamping device 10 due to unknown temperatures no longer plays a role. Such clamping devices 10 could thus even be used in the fluid channels, since the clamping device 10 made of carbon fiber composite material would not elongate when heated gas flows in.

The compensation of the thermal changes in length of the individual cells 4 is thus taken over by the adjusting element 11, preferably completely taken over by it. This can be active, for example in the case of an actuating element 11 designed as a piezo actuator, or else passive, for example in that the actuating element 11 is integrated into the clamping device 10 and has a significantly greater coefficient of thermal expansion than the individual cells 4.

Claims

Expectations

1. Cell stack (1) with several individual cells (4), the individual cells (4) being clamped between two end plates (2, 3) by means of at least one clamping device (10), characterized in that the at least one clamping device (10) consists of a Tensioning material, whereby the tensioning material has a coefficient of thermal expansion <2.5 * 10 ^L -6 m / (m * K).

2. cell stack (1) according to claim 1, characterized in that the tensioning material has a modulus of elasticity> 90,000 N / mm ² .

3. cell stack (1) according to claim 2, characterized in that the tensioning material is designed as a carbon fiber composite material.

4. cell stack (1) according to any one of claims 1 to 3, characterized in that the at least one clamping device (10) is designed as a tie rod.

5. cell stack (1) according to any one of claims 1 to 3, characterized in that the at least one clamping device (10) is designed as a housing.

6. cell stack (1) according to any one of claims 1 to 3, characterized in that the at least one clamping device (10) is designed as a clamping band.

7. cell stack (1) according to claim 6, characterized in that the at least one clamping device (10) has a reduced cross section in an expansion region (10a).

8. cell stack (1) according to claim 7, characterized in that the reduced cross section is achieved by means of a waist.

9. cell stack (1) according to claim 7 or 8, characterized in that the reduced cross section is achieved by means of a thinning.

10. cell stack (1) according to any one of claims 1 to 9, characterized in that the cell stack (1) has an adjusting element (11).

11. cell stack (1) according to claim 10, characterized in that the adjusting element (11) is designed as a piezo actuator.

12. cell stack (1) according to claim 10 or 11, characterized in that the adjusting element (11) has a spacer (11a).

13. cell stack (1) according to any one of claims 10 to 12, characterized in that the adjusting element (11) is integrated in the clamping device (10).

14. cell stack (1) according to claim 13, characterized in that the adjusting element (11) has a greater heat transfer coefficient than the individual cells (4).

15. cell stack (1) according to any one of claims 10 to 12, characterized in that the adjusting element (11) is integrated in the cell stack (1).

16. cell stack (1) according to any one of claims 1 to 15, characterized in that the cell stack (1) has a fluid channel (15) for supplying a fluid to the individual cells (4), wherein the Clamping device (10) is arranged at least partially in the fluid channel (15).

17. Motor vehicle with a cell stack (1) designed as a fuel cell stack according to one of the preceding claims.