Compression device for electrochemical cell stacks with a non-linear spring device

Abstract

The invention relates to a compression device (10) for exerting a compressive force on an electrochemical cell stack (12), comprising a first compression plate (24) and a second compression plate (26), said first and second compression plates being displaceable towards each other along a compression axis (30) for compressing an electrochemical cell stack received between them, further comprising a spring device (36), comprising at least one spring member (38), for exerting a spring force on the first compression plate, said spring force acting along the compression axis, wherein the spring device has a non-linear force-deflection behaviour. The invention also relates to a cell assembly comprising such a compression device and an electrochemical cell stack, and to methods of manufacturing a compressed electrochemical cell stack.

Description

[0001] Title: Compression device for electrochemical cell stacks with a non-linear spring device

[0002] Specification

[0003] The invention relates to the field of electrochemical cell stacks, in particular, fuel cell stacks and electrolyser cell stacks. More specifically, the invention relates to a compression device for exerting a compressive force on an electrochemical cell stack and to a method of manufacturing a compressed electrochemical cell stack.

[0004] Fuel cells and electrolyser cells are examples of electrochemical cells. Fuel cells are energy conversion devices that allow for conversion of electrochemical fuel (e.g. H2) to electricity. Electrolyser cells may be considered fuels cells running in reverse mode, i.e. using electricity to decompose a compound into its constituent parts, for example, water into hydrogen and oxygen. Reversible cells are capable of operating in both modes. Such electrochemical cells typically comprise electrochemically active layers that may be configured to allow for conversion of electrochemical fuel to electricity (fuel cells) or for decomposing a compound into its constituent parts using electricity (electrolyser cells).

[0005] Typically, multiple of such cell units are stacked upon one another in a stacking direction to form a "stack" of cell units, with gaskets being interposed between adjacent cell units to avoid loss of fluid. Optional end plates may be disposed to opposing ends of the cell stack. In addition, electrical insulators, e.g. in the form of insulation plates, may be disposed between adjacent cell units and / or between the cell units and the optional end plates.

[0006] During manufacture, the cell stack is usually compressed in a compression device and subsequently subjected to stack conditioning processes in the compressed state. For example, the compressed stack may be subject to hot conditioning or baking processes. The inventors have found that during such condition processes, but also during final operation of the cell stack, variations in the compressive force acting on the cell stack may occur due to shrinkage of the cell stack, e.g. due to relaxation of the gaskets or insulators exposed to elevated temperatures. It is an object of the present invention to improve the compression of electrochemical cell stacks.

[0007] According to the invention there is provided a compression device according to claim 1. The compression device is configured to exert a compressive force on an electrochemical cell stack, preferably fuel cell stack or electrolyser cell stack. Thus, the compression device is configured for compressing an electrochemical cell stack. The compression device comprises a first compression plate (also referred to as pressure distribution plate) and a second compression plate (also referred to as pressure distribution plate). The first compression plate and the second compression plate are displaceable towards each other along a compression axis for compressing an electrochemical cell stack received between said first compression plate and said second compression plate, i.e. for exerting a compression force on the cell stack along the compression axis.

[0008] The compression device comprises a spring device. The spring device is configured to exert, i.e. apply, a spring force on the first compression plate along the compression axis, preferably such that the spring force of the spring device is transmitted to the cell stack via the first compression plate. The spring device comprises at least one spring member. Preferably, the or each spring member is configured to exert, i.e. apply, a spring force on the first compression plate along the compression axis. The spring device, preferably the or each spring member, is deflectable, preferably compressible, along the compression axis. The spring device has a non-linear force-deflection, preferably force-compression, characteristic. Specifically, the spring device is configured such that a characteristic curve representing the progression of the spring force exerted by the spring device as a function of the deflection, preferably compression, of the spring device along the compression axis (i.e. a force-compression characteristic of the spring device) has a first section (first compression regime) having a first gradient and a second section (second compression regime) having a second gradient, wherein said second gradient is larger than said first gradient. The first gradient may be an average gradient taken within the first section. The second gradient may be an average gradient taken within the second section. Said averages may be over the whole of the section or a portion thereof. Thus, the spring device, preferably the at least one spring member, has a non-linear force-deflection characteristic, said force-deflection characteristic having a first section having a first gradient and a second section having a second gradient, wherein said second gradient is larger than said first gradient. The second section is at higher deflection than the first section. Preferably, the second section adjoins the first section towards higher deflection / compression values. The proposed compression device allows for reliable and targeted compression of electrochemical cell stacks. Specifically, the proposed spring device having a non-linear force-deflection characteristic makes it possible to compensate for stack shrinkage, for example due to relaxation of gaskets and / or insulators therein, when exposed to elevated temperatures. The two sections of the force-deflection curve having different gradients allow for high compression load prior to any stack conditioning processes (due to the large gradient in the second section), and lower post-conditioning load targeted at appropriate value (due to the relatively small gradient in the first section). Advantageously, the relatively small gradient in the first section allows for appropriately targeted compression load even if the degree of shrinkage varies or changes over time. For example, in a method of manufacturing a compressed cell stack, the spring device may initially be deflected, preferably compressed, to a deflection in the second section, thus leading to relatively high compressive forces and thus reliable compression of the gaskets and / or insulators. When subjected to stack conditioning processes, the stack may undergo relaxation, e.g., due to the gaskets and / or insulators therein, such that deflection of the spring device decreases. The relatively high gradient in the second section ensures that even low shrinkage is sufficient to reach into the first section, in which further shrinkage (e.g. during further conditioning processes or during cell stack operation), and thus further changes in the deflection, do not lead to significant change in compression force.

[0009] As used herein, the term "gradient" means the change in the spring force divided by the corresponding change in the deflection (compression) between two distinct points on the characteristic curve. In particular, the term "gradient" refers to the gradient of a tangent line to the characteristic curve at a specific deflection / compression value. The term "gradient of a section" preferably means an average gradient in said section, i.e. an average value of the gradients in the respective section. The gradient may correspond to the spring constant of the spring device. The gradient may be constant within a respective section. The gradient may vary within a respective section. The gradient in the first and / or second section may be an average gradient, for example taken over a deflection, preferably compression, range of at least 0.2 mm, preferably at least 0.5 mm, more preferably at least 1.0 mm in the respective section. Said average may be taken over a deflection, preferably compression, range of at most 3.0 mm, preferably at most 2.0 mm, more preferably at most 1.0 mm. As used herein, the term "deflection" of the spring device or the spring member refers to a deformation of the spring device, in particular of the at least one spring member, that leads to spring energy being stored in the spring device or spring member. In some embodiments, such deformation may be a compression of the spring device, in particular of the at least one spring member. Thus, the non-linear force-deflection characteristic may be a non-linear force-compression characteristic. For example, the at least one spring member may be a compression spring.

[0010] The first and second sections may be contiguous, such that as the spring device is deflected, the deflection passes from the first section to the second section.

[0011] In some examples, the gradient may be constant within each of said first and second sections (i.e. the characteristic curve may have a linear course in each of said first and second sections). In such examples, the gradient may change abruptly at the transition from the first section to the second section.

[0012] In other examples, there may be a gradual change of gradient between the sections (the deflection range of that gradual change may be less than 30% of the deflection range of the first section, preferably less than 20%, more preferably less than 10%). Outwith the deflection range of the gradual change, the sections may have a reasonably constant gradient (e.g., varying by less than 25%, preferably less than 10%). An average gradient for a section may be taken to be that gradient in a portion of the section that is reasonably constant.

[0013] The spring device may comprise or consist of a single spring member. In such embodiments, preferably said spring member has the non-linear force-deflection (force-compression) characteristic described above.

[0014] Alternatively, the spring device may comprise more than one spring member. In such embodiments, the plural spring members may interact to provide the non-linear force-deflection (force-compression) characteristic - without necessarily having such a non-linear force-deflection (force-compression) characteristic themselves. In preferred embodiments, each spring member has a non-linear force-deflection, preferably forcecompression, behaviour in that a characteristic curve representing the progression of the spring force exerted by the spring member as a function of the deflection, preferably compression, of the spring member along the compression axis has a first section having a first gradient and a second section at higher deflection than the first section having a second gradient, said second gradient being larger than said first gradient.

[0015] The or each spring member may comprise a single spring. In such embodiments, the single spring may have a non-linear force-deflection characteristic. Preferably, the or each spring member comprises a plurality of springs. In some embodiments, the or each spring member comprises or consists of a stack of springs. Each stack of springs may comprise a plurality of springs stacked upon one another along the compression axis. In some embodiments, the spring device may comprise springs having different spring constants.

[0016] Preferably, the spring device comprises more than one spring member, more preferably at least four spring members. The spring members may be arranged at corners of the first compression plate.

[0017] In preferred embodiments, the first compression plate is generally rectangular or square-shaped having four corners. In such embodiments, a spring member may be located at or adjacent to each corner.

[0018] Preferably, the first compression plate and the second compression plate are parallel to each other. Preferably, the first compression plate and the second compression plate each extend in a plane perpendicular to the compression axis. The first compression plate may be a top plate of the compression device. The second compression plate may be a base plate of the compression device. The first compression plate and / or the second compression plate may be formed from metal. The first compression plate and / or the second compression plate may be flat or planar.

[0019] The first compression plate and the spring device may be connected to form a unit. Thus, the first compression plate and the spring device may be handled as single piece. Advantageously, this may facilitate a compression process (see below). In some embodiments, the compression device comprises a third compression plate located to a side of the first compression plate opposite the second compression plate (in other words a first face of the first compression plate faces the second compression plate and a second, opposite, face of the first compression plate faces the third compression plate). In such embodiments, the spring device, preferably the at least one spring member, may be located between the first compression plate and the third compression plate. Such a configuration eases a compression process, in particular in case of plural spring members. Specifically, the third compression plate allows to compress the spring members homogeneously by applying a load to the third compression plate, e.g. via a mechanical press. The first compression plate, the spring device, and the third compression plate may be connected to form a unit. The spring device may connect the first and the third compression plate. The first compression plate and the third compression plate may be displaceable towards each other along the compression axis, preferably against the bias of the spring device. Thus, the spring device may be configured to bias the first compression plate and the third compression plate away from each other.

[0020] The compression device may be a permanent part that remains with the electrochemical cell stack postmanufacture, e.g., as a cell stack assembly, e.g., during use of the cell stack assembly (e.g. when converting fuel into electricity or decomposing a compound into its constituent parts).

[0021] Preferably, the compression device is intended to be applied only temporarily during the manufacture of a cell stack. Thus, the compression device may not be a permanent part of the electrochemical cell stack during its final use. Specifically, the compression device may be an aid during the manufacturing process of the cell stack for holding the cell stack in compression during certain manufacturing steps, such as stack conditioning processes.

[0022] Preferably, the spring device is configured such that the second gradient is at least two times larger than the first gradient, preferably at least three times, more preferably at least five times. This further improves the above-described technical effects.

[0023] In some embodiments, the first section extends over a first deflection range, preferably compression range, and the second section extends over a second deflection range, preferably compression range, wherein the first deflection range is at least two times larger than the second deflection range, preferably at least three time larger, more preferably at least five times larger. This further improves the above-described technical effects. Specifically, as the first deflection range covers a relatively large span of deflection, even relatively large variations in deflection, e.g. due to different grades of relaxation, do not lead to significant change in compression force. This also allows to use the same compression device for compression of cell stacks having a different number of cell units, which will lead to variations in overall relaxation.

[0024] As used herein, the term "deflection range" or "compression range" refers to the difference between the maximum deflection / compression value of a respective section and the minimum deflection / compression value of said section.

[0025] In some embodiments, the first section extends over a (first) deflection range, preferably compression range, of at least 2.0 mm, preferably at least 3.0 mm. This range has proven advantageous with regards to the above-described effects.

[0026] In some embodiments, the second section extends over a (second) deflection range, preferably compression range, of at most 3.0 mm, preferably at most 2.0 mm, more preferably at most 1.0 mm, more preferably at most of 0.5 mm.

[0027] Preferably, the spring device is configured such that an average spring force, and thus compressive load on the stack, in the second section is between 2 and 8 times greater than in the first section, preferably between 3 and 6 times greater.

[0028] In some embodiments, the spring device, preferably the or each spring member, has a progressive characteristic in the second section. That is to say, in the second section, a second derivative of the force-deflection curve (characteristic curve) may be positive. This aids obtaining relatively high compression forces at relatively low deflection.

[0029] In preferred embodiments, the first section is a plateau section. Preferably, said plateau section is characterised in that within said plateau section, i.e. for all deflection values within the first section, the spring force changes by at most 10%, preferably by at most 5%. Thus, variations in the deflection of the spring device, e.g. due to different stack shrinkage, do not lead to significant changes in the compressive load on the stack. Preferably, within said first section, the spring force changes by at most 10%, preferably by at most 5% about a target spring force. Preferably, said target spring force is between 1 and 100 kN, more preferably between 1 and 50 kN.

[0030] In preferred embodiments, the characteristic curve has a third section (third compression regime) at lower deflection, preferably lower compression, than the first section. Preferably, the third section adjoins the first section at a low deflection value of the first section (in other words the third and first sections are contiguous, as may be the first and second sections, such that as the spring device is deflected, the deflection passes from the third section, to the first section, to the second section. It will be understood that there may be a gradual change of gradient between the sections).

[0031] Preferably, the third section has a third gradient that is larger than the first gradient.

[0032] Preferably, the spring device, more preferably the or each spring member, has a digressive characteristic in the third section. That is to say, in the third section, a second derivative of the force-deflection curve (characteristic curve) may be negative.

[0033] In preferred embodiments, the spring device, preferably the or each spring member, is configured such that a characteristic curve representing the spring force exerted by the spring device, preferably by the spring member, over deflection (i.e. its force-deflection characteristic) has a digressive section (third section), a plateau section (first section, preferably linear or horizontal section), and a progressive section (second section) in its course from lower to higher deflection.

[0034] The spring member may take various configurations. The spring member or one of the spring members may be a compression spring. In preferred embodiments, the or each spring member comprises at least one disc spring (also referred to as Belleville springs or Belleville washers). In such embodiments, a nonlinear force-compression characteristic may be provided by adjusting a material thickness and a clear height of the disc spring. In some embodiments, the or each spring member comprises a stack of disc springs. Preferably, the compression device further comprises a clamping device (or tensioning device), said clamping device being configured to deflect, preferably compress, the spring device, preferably the at least one spring member, along the compression axis and / or to hold the spring device, preferably the at least one spring member, in the deflected, preferably compressed, state (which in turn holds the electrochemical cell stack in compression). Thus, the clamping device may be configured to compress the spring device and maintain said compression. Alternatively, the spring device may already be compressed, e.g. by applying a compressive load to the third compression plate. In such embodiments, the clamping device may be configured to hold the spring device in the deflected, preferably compressed, state (which in turn holds the electrochemical cell stack in compression).

[0035] In embodiments comprising a third compression plate, wherein the spring device is located between the third compression plate and the first compression plate, the clamping device may be configured to limit axial displacement of the third compression plate and the second compression plate away from each other along the compression axis. Thus, the clamping device may be a locking device.

[0036] Preferably, the compression device further comprises a linear guide device for guiding an axial displacement of the first compression plate, and preferably of the at least one spring member, along the compression axis. The linear guide device may also be configured to guide an axial displacement of the second compression plate along the compression axis, preferably independently from an axial displacement of the first compression plate and / or the at least one spring member.

[0037] The linear guide device may comprise one or more linear guides. The one or more linear guides may extend along the compression axis from the second compression plate to the first compression plate, and optionally to the third compression plate. The one or more linear guides may extend perpendicular to an extent of the first and second compression plates. The one or more linear guides may be guiding rods, tensioning rods, bolts, or the like.

[0038] The one or more linear guides may extend through the first compression plate along the compression axis. For this, the first compression plate may comprise respective openings for receiving the one or more linear guides or guiding rods. In preferred embodiments, the or each spring member of the spring device surrounds a respective linear guide. More specifically, the linear guide may extend through a respective central opening of the spring member, e.g., through a central opening of a disc spring or disc springs of the spring member.

[0039] In some embodiments, the one or more linear guides or guiding rods may each comprise a threaded portion or may be threaded rods. In such embodiments, the clamping device may comprise one or more nuts or clamps configured to cooperate with said threaded portion or threaded rod for adjusting deflection, preferably compression, of the spring device, preferably of the at least one spring member, along the compression axis (e.g. by loosening or tightening of the nut or clamp) and / or to hold the spring device, preferably the at least one spring member, in the deflected, preferably compressed, state. In embodiments comprising a third compression plate, the clamping device may comprise one or more nuts or clamps configured to cooperate with said threaded portion or threaded rod for limiting axial displacement of the third plate along the compression axis in a direction away from the second compression plate.

[0040] The compression device may be used to compress electrochemical cell stacks of various kinds.

[0041] Preferably, the electrochemical cell stack comprises a plurality of cell units (also referred to as cell repeat units) stacked upon one another along a stacking direction.

[0042] The cell units may be fuel cell units, electrolyser cell units or reversible cell units. The cell units may be metal-supported electrochemical cell units. The cell units may be solid oxide fuel cell units or solid oxide electrolyser cell units. The cell units each may comprise multiple layers, preferably including a cell layer comprising electrochemically active layers, and, optionally, an interconnector plate. The electrochemically active layers may comprise a fuel electrode layer, an electrolyte layer, and an air or oxidant electrode layer. The electrochemically active layers may be deposited or coated (e.g. as thin coatings or films) on and supported by a support plate of the cell layer, e.g. by a metal support plate, such as a metal foil, thereby forming metal-supported electrochemical cell unit.

[0043] Preferably, the electrochemical cell stack comprises gaskets interposed between the cell units. In some embodiments, the cell units may each comprise at least one, preferably two or more fluid ports, preferably in the form of through-holes, said fluid ports allowing fluid to enter and exit the cell units (an internal fluid volume thereof). Preferably, the fluid ports are in fluidic communication with the electrochemically active layers of the cell units (e.g. via respective fluid channels). The gaskets may surround said fluid ports of the cell units.

[0044] The gaskets may be formed from a material suitable for a high temperature sealing gasket, for example a mineral-based material, vermiculite material or a rubber material.

[0045] In some embodiments, the electrochemical cell stack further comprises two end plates located at opposite ends of the stack of cell units along the stacking direction. Thus, the cell units may be interposed between two end plates. That is to say, the cell stack may comprise a first end plate located at a first end of the cell stack, and a second end plate located at an opposite second end of the cell stack.

[0046] The electrochemical cell stack may further comprise one or more electrical insulators, for example one or more insulation plates located between the end plates and the cell units. The insulators may be formed from a ceramic material. The insulator may be formed form a vermiculite material.

[0047] The invention also relates to a cell stack assembly comprising an electrochemical cell stack and a compression device as described above. The compression device comprises a first compression plate, a second compression plate, and a spring device configured as described above (i.e., having a non-linear force-deflection characteristic). The optional features and advantages explained above in connection with the compression device and the electrochemical cell stack themselves are also applicable to the cell stack assembly. Preferably, the electrochemical cell stack comprises a plurality of cell units stacked upon one another along a stacking direction. Preferably, the electrochemical cell stack is held in a compressed state between the first compression plate and the second compression plate of the compression device. Preferably, the electrochemical cell stack is interposed between the first compression plate and the second compression plate such that the stacking direction is parallel to the compression axis of the compression device.

[0048] As set out above, the compression device preferably is a device applied only temporarily during the manufacture of the final compressed cell stack. Thus, the cell stack assembly may be an intermediate product existing only temporarily in the course of manufacturing a compressed cell stack. In other embodiments, the compression device may remain with the electrochemical cell stack during its final use. Thus, the cell stack assembly may be the actual operating unit. Such a cell stack assembly may include the electrochemical cell stack and the compression device. The first and second compression plates may form end plates disposed to opposing ends of the electrochemical cell stack. The spring device may be held in a deflected state by the clamping device to maintain compression in the electrochemical cell stack (between the first and second compression plates).

[0049] In cases where the compression device comprises first, second, and third compression plates, the second and third compression plates may form end plates of the electrochemical cell stack. The third compression plate and the second compression plate may be held at a fixed distance from each other along the compression axis such that the spring device located between the third compression plate and the first compression plate is held in a deflected state to maintain compression in the electrochemical cell stack (between the first and second compression plates). The third compression plate and the second compression plate may be held at a fixed distance from each other along the compression axis by the clamping device. Alternatively or additionally, the first and second the cell stack assembly may further comprise a housing. The housing may be attached to the second and third compression plates to maintain a spacing therebetween.

[0050] The invention also relates to methods of manufacturing a compressed cell stack using a compression device as described above. Thus, the disclosure also relates to the use of a compression device as described above for compressing a cell stack.

[0051] According to a first aspect, the method comprises providing a compression device as described above. The method further comprises a step of providing an electrochemical cell stack between the first compression plate and the second compression plate of the compression device. For this, the first compression plate may be held at distance from the second compression plate such that a cell space for receiving the cell stack is provided in between. After that, the cell stack is compressed using the compression device. Specifically, the method comprises a step of adjusting said first and second compression plates towards each other along the compression axis to compress the electrochemical cell stack. The method further comprises a step of tensioning the spring device, preferably the at least one spring member, by deflecting, preferably compressing, the spring device, preferably the at least one spring member, to a deflection, preferably compression, within the second section of the characteristic curve of the spring device. The method may further comprise holding the spring device in said deflected, preferably compressed, state.

[0052] The step of adjusting the first and the second compression plates towards each other along the compression axis may comprise a step of applying load to the first compression plate in a direction towards the second compression plate, preferably using a press.

[0053] In some embodiments, the compression device comprises a clamping device configured to deflect, preferably compress, the spring device along the compression axis and to hold the spring device in the deflected, preferably compressed, state (see above). In such embodiments, the step of deflecting the spring device may comprise compressing the spring device, preferably the at least one spring member, between a clamping member of the clamping device and the first compression plate.

[0054] In embodiments, wherein the compression device comprises a linear guide device having linear guides having a threaded portion, and the clamping device comprises a nut that is interacting with a threaded portion, the steps of deflection the spring device, in particular the at least one spring member, may be provided by tightening the nut.

[0055] According to a second aspect, the method comprises providing a compression device as described above, said compression device comprising a third compression plate located on a side of the first compression plate opposite to the second compression plate, wherein the spring device, preferably the at least one spring member, is interposed between the first compression plate and the third compression plate, and comprising a clamping device configured to limit axial displacement of the third compression plate along the compression axis in a direction away from the second compression plate. The method further comprises a step of providing an electrochemical cell stack between the first compression plate and the second compression plate of the compression device. For this, the first compression plate may be held at a distance from the second compression plate such that a cell space for receiving the cell stack is provided in between. After that, the cell stack is compressed using the compression device. Specifically, the method comprises a step of adjusting said first and second compression plates towards each other along the compression axis to compress the electrochemical cell stack by applying load to the third compression plate along the compression axis, preferably using a press, such that the spring device, preferably the at least one spring member, located between the first and third compression plates, is deflected, preferably compressed, to a deflection, preferably compression, within the second section of the characteristic curve of the spring device. To maintain the stack compression, the method comprises a step of activating the clamping device of the compression device to block axial displacement of the third compression plate and the second compression plate away from each other along the compression axis. For example, in embodiments, in which the clamping device comprises at least one nut, activating the clamping device may comprise tightening the at least one nut.

[0056] After compressing the cell stack by the compression device, the cell stack may be fixed in the compressed state. Thus, the method may comprise a step of fixing the cell stack such that the cell stack is held in the compressed state even after removing the compressive load. In some implementations, said fixing may comprise applying a (single or multi-piece) housing to the cell stack after compression such that the cell stack is held in its compressed state by the housing. For example, the fixing step may include a step of welding a housing to the optional first and second end plates of the cell stack.

[0057] Alternatively or in addition, fixing members, such as clamps, compression rods, plates or springs or bolts, may be applied to the cell stack in order to maintain the cell stack in the compressed state.

[0058] The method may, preferably after the fixing step, further comprise a step of removing the cell stack from the compression device. This may include a step of releasing the compressive load applied by the compression device. For this, the clamping device may be deactivated, e.g. by at least slightly loosening the nuts, to enable axial displacement of the first compression plate away from the second compression plate along the compression axis.

[0059] In some embodiments, the method may further comprise one or more stack conditioning processes performed on the compressed electrochemical cell stack.

[0060] The stack conditioning processes (e.g. the hot conditioning or baking step) may be performed on the cell assembly comprising the (compressed) electrochemical cell stack and the compression device. In some embodiments, the stack conditioning processes comprise heat-treating the (compressed) electrochemical cell stack. For example, the method may comprise hot conditioning the electrochemical cell (e.g. on a test stand producing power or by circulating warm fluids through the stack). Alternatively or additionally, the method may comprise baking the compressed electrochemical cell stack, e.g. in an oven or hotbox.

[0061] As set out above, the electrochemical cell stack preferably comprises at least one gasket in the compression path, in particular at least one gasket between each pair of adjacent cell units. Thus, the step of providing the electrochemical cell stack may comprise providing at least one gasket, preferably between each pair of adjacent cell units.

[0062] Furthermore, the electrochemical cell stack may comprise at least one electrically insulating plate, e.g. mica plate. Thus, the step of providing the electrochemical cell stack may comprise providing at least one electrically insulating plate. Preferably, the electrically insulating plate or one of the electrically insulating plates is disposed to an end of the stack of cell units. In embodiments, comprising one or more end plates, preferably an insulating plate is provided between the stack of cell units and one or both of the end plates.

[0063] Further embodiments are derivable from the following description and the drawings.

[0064] In the drawings:

[0065] Fig. 1 shows a schematic side view of a first embodiment of a compression device and a cell stack;

[0066] Fig. 2 shows a schematic side view of a second embodiment of a compression device and a cell stack;

[0067] Fig. 3 shows a diagram illustrating an exemplary force-deflection characteristic of a spring device according a first implementation;

[0068] Fig. 4 shows a diagram illustrating an exemplary force-deflection characteristic of a spring device according a second implementation; Fig. 5 shows a flowchart for illustrating a method of manufacturing a compressed cell stack using a compression device according to Figure 1; and

[0069] Fig. 6 shows a flowchart for illustrating a method of manufacturing a compressed cell stack using a compression device according to Figure 2.

[0070] Repeat use of reference symbols in the present specification and drawings is intended to represent the same or analogous features or elements.

[0071] Figure 1 schematically shows an example implementation of a compression device 10 according to a first embodiment. The compression device 10 is configured for exerting a compressive force on an electrochemical cell stack 12 (hereinafter referred to as cell stack).

[0072] The cell stack 12 comprises a plurality of cell units 14 that are stacked upon one another along a stacking direction 16. As set out above, the cell units 14 may, for example, be fuel cell units, electrolyser cell units or reversible cell units. The compression device 10, however, may be used to compress electrochemical cell stacks 12 of different kinds.

[0073] In the example, the cell stack 12 further comprises a first end plate 18 and a second end plate 20 disposed to opposite ends of the cell stack 12. Between the end plates 18, 20 and the cell units 12, optional insulation plates 21, e.g. mica plates, may be disposed.

[0074] The cell stack 12 further comprises gaskets 22 that are interposed between adjacent cell units 14. The gaskets 22 may surround respective fluid ports (not shown) of the individual cell units 14. For example, the gaskets 22 may take the form of sealing rings that surround respective through-holes formed in the cell units 14.

[0075] The compression device 10 comprises a first compression plate 24 and a second compression plate 26, which define a compression space 28 therebetween for receiving the cell stack 12.

[0076] The first compression plate 24 and the second compression plate 26 are displaceable towards each other along a compression axis 30 to exert a compression force on the cell stack 12. As shown in Figure 1, preferably the compression axis 30 and the stacking direction 16 are parallel or identical. Preferably, the first compression plate 24 and the second compression plate 26 extend perpendicular to the compression axis 30.

[0077] The compression device 10 further comprises a linear guide device 32 for guiding an axial displacement of the first compression plate 24 and / or the second compression plate 26 along the compression axis 30. In the example, the linear guide device 32 comprises a plurality of linear guides 34, preferably one linear guide 34 adjacent to each corner of the first and second compression plates 24, 26.

[0078] As can be seen from Figure 1, the linear guides 34 extend along the compression axis 30 from the second compression plate 26 to the first compression plate 24. More specifically, the linear guides 34 extend through the first compression plate 24. For this, the first compression plate 24 may comprise respective openings (not shown).

[0079] The linear guides 34 may take the form of guiding rods, tensioning rods, or bolts.

[0080] The compression device 10 further comprises a spring device 36 for exerting a compressive force on the cell stack 12 along the compression axis 30 (indicated by the arrows in Fig. 1). Specifically, the spring device 36 comprises a plurality of spring members 38 that are arranged to exert a spring force on the first compression plate 24.

[0081] In the example, each linear guide 34, and thus each corner of the first compression plate 24, is associated with one spring member 38. Thus, in the example, the spring device 36 comprises four spring member 38. In other embodiments, the spring device 36 may comprise greater or fewer spring members 38. For example, the spring device may comprise a plurality of spring members 38 that are distributed across the first compression plate 24.

[0082] Each spring member 38 may comprise one or more springs 40 (only schematically shown in Figure 1). In the specific example, each spring member 38 comprises plural springs 40 that are stacked upon one another along the compression axis 30. Thus, the spring members 38 may take the form of spring stacks. Preferably, the springs 40 are disc springs. In the example, the spring members 38 are located on the side of the first compression plate 24 opposite the second compression plate 26. Specifically, the spring members 38 are provided around said linear guides 34 (see Fig. 1). For example, the linear guides 34 may extend through a central opening of the springs 40 of the spring members 38.

[0083] In some embodiments, the first compression plate 24 and the spring members 38 may be connected to form a unit.

[0084] As will be described in detail below with respect to Figures 3 and 4, the spring device 36, in particular each spring member 38, has a non-linear force-compression characteristic.

[0085] The compression device 10 further comprises a clamping device 42. The clamping device 42 is configured to compress the spring members 38 along the compression axis 30 and to hold the spring device 38 in said compressed state.

[0086] In the example, the locking device 42 comprises nuts 44 that cooperate with threaded portions (not shown in detail) of the linear guides 34. Alternatively, the linear guides 34 may be configured as threaded rods. Thus, by screwing the nuts 44 on the threaded portions of the linear guides 34, a compressive force may be exerted on the spring members 38, thus storing spring energy in the spring members 38.

[0087] Figure 2 shows an example implementation of a compression device 10 according to a second embodiment. The compression device 10 of this embodiment is generally identical to the compression device 10 of Figure 1, except that a third compression plate 46 is provided on the side of the first compression plate 24 opposite the second compression plate 26. To avoid repetition, aspects of Figure 2 that are generally the same as shown in Figure 1 will not be described again, and like reference numerals are used to describe like features.

[0088] In the example of Figure 2, the spring device 36 is arranged between the first compression plate 24 and the third compression plate 46. Optionally, the first compression plate 24, the spring members 38 of the spring device 36, and the third compression plate 46 may be connected to form a unit. In this example, the clamping device 42 (nuts 44) are configured to limit axial displacement of the third compression plate 46 along the compression axis 30 in a direction away from the second compression plate 26. Thus, the clamping device 42 is configured to set a maximum distance between the third compression plate 46 and the second compression plate 26.

[0089] In the compressed state, the third compression plate 46 and the second compression plate 26 are held at a fixed distance from each other along the stacking direction 16 / compression axis 30 such that the cell stack 12 is held in compression due to the spring force provided by the spring device 36 (applied to the cell stack 12 via the first compression plate 24).

[0090] In this case, compression may be applied to the spring device (and to the cell stack 12 via the first compression plate 24) by a press (not shown in the Figure) configured to act between the third compression plate 46 and the second compression plate 26. Once a desired compression is reached, the clamping device 42 may be used to maintain the compression, the press released, and the assembly (compression device and cell stack) removed from the press.

[0091] As set out above, in both embodiments, the spring device 36 comprises a non-linear force-compression characteristic.

[0092] Figure 3 shows an exemplary force-compression curve 48 of the spring device 36. In this example, the force-compression curve comprises - along its course from lower to higher compression - a first section

[0093] I having a first gradient 50, and a second section II having a second gradient 52, wherein the second gradient 52 is larger than the first gradient 50.

[0094] As can be seen from Figure 3, the first section I extends over a first compression range 54, and the second section I extends over a second compression range 56, wherein the first compression range 54 is at least two times larger than the second compression range 56, in the specific example about 6 times larger.

[0095] Figure 4 shows a force-compression curve 48 of a further exemplary spring device 36. In this example, the force-compression curve 48 comprises - along its course from lower to higher compression - a digressive section (third section III), a plateau section (first section I) and a progressive section (second section II).

[0096] As can be seen from Figure 4, a gradient 50 in the first section I is smaller than a gradient 52 in the second section II and a gradient 58 in the third section III. As can be seen from Figure 4, an average gradient in the first section I is smaller than an average gradient in the second section II and an average gradient in the third section III. The gradient in each section may be an average gradient taken over a portion of that section, for example taken over a deflection, preferably compression, range of at least 0.2 mm, preferably at least 0.5 mm, more preferably at least 1.0 mm in the respective section. Said average may be taken over a deflection, preferably compression, range of at most 3.0 mm, preferably at most 2.0 mm, more preferably at most 1.0 mm. The first and second sections may be contiguous, such that as the spring device 36 is deflected, the deflection passes from (the optional third section III to the first section I and from) the first section I to the second section II.

[0097] It will be understood that there may be a gradual (i.e., continuous: mathematically differentiable) change of gradient between the sections. The deflection range of that gradual change may be less than 30% of the deflection range of the first section, preferably less than 20%, more preferably less than 10%. Nonetheless, outwith the deflection range of the gradual change, the sections may have a reasonably constant gradient (e.g., varying by less than 25%, preferably less than 10%). An average gradient for a section may be taken to be that gradient in a portion of the section that is reasonably constant (for example over the deflection, preferably compression, ranges mentioned above).

[0098] Alternatively or additionally, the gradient of a particular section (particularly the first section I) may be taken to be a gradient (i.e., tangent to the curve) at a deflection (or compression value) where the second derivative of the force-compression curve is zero (equivalently, the deflection / compression where the first derivative of the gradient is zero).

[0099] Such a force-compression characteristic may, for example, be achieved by tuning a material thickness and a clear height of a disc spring and use of more than one disc spring.

[0100] In the following, a method of manufacturing a compressed cell stack using a compression device according to the first embodiment (Figure 1) will be described with reference to Fig. 5. In a first step (see block 100 in Figure 5), a compression device 10 as described above is provided and a cell stack 12 is placed in the compression space 28 between the first compression plate 24 and the second compression plate 26 of the compression device 10.

[0101] For this, the first compression plate 24 may be held at distance from the second compression plate 26 (e.g. by a user). Alternatively, the first compression plate 24 may initially be removed from the linear guides 34, and the cell stack 12 may be arranged on the second compression plate 26. The first compression plate 24 and the spring members 28 may then be placed on the cell stack 12 such that the linear guides 34 extend through the first compression plate 24 and the spring members 28.

[0102] In a next step (see block 102 in Fig. 5), the cell stack 12 is compressed along the compression axis 30. For this, the first compression plate 24 is adjusted towards the second compression plate 26 along the compression axis 30 (guided by the linear guides 34), for example, by applying load to the first compression plate 24 using a mechanical press 60.

[0103] After reaching a predefined compression grade of the cell stack 12, the spring members 28 are compressed by tightening the nuts 44 (see block 104 in Fig. 5). Specifically, the spring members 28 are compressed to a compression within the second section II of the characteristic curve 48 of the spring device (illustrated by the "X" in the exemplary force-compression curve 48 shown on the right in Figure 5, block 104).

[0104] After that, the load applied to the first compression plate 24 (e.g. via the press 60) is removed. In this configuration, the applied compression force is maintained in the cell stack 12 due to the spring force provided by the spring device 36.

[0105] As set out above, the method may further comprise stack conditioning processes. In the example of Figure 5, the method comprises a baking step (see block 106 in Figure 5). Specifically, the cell stack 12 and the compression device 10 are heat-treated in an oven 62 or hotbox, for example at temperatures between 300 °C and 700 °C. As illustrated by the force-compression diagram depicted on the right of block 106 in Figure 5, during the baking step, shrinkage of the cell stack 12 and thus reduction in compression of the spring members 28 may occur, e.g. due to relaxation of the insulation plates 21 and / or the gaskets 22. Specifically, the spring device 36 is configured such that shrinkage occurring during the stack baking process is sufficient to change compression of the spring device 36 from the second section II to the first section I (illustrated by the "X"). Due to the relatively low gradient in the first section I, further shrinkage of the cell stack 12 (e.g. due to further relaxation of the gaskets 22 during further processing steps or during normal use of the cell stack 12) does not lead to significant change in spring force and thus compression force. As set out above, this allows to maintain a relatively constant compression force in the cell stack 12.

[0106] The method may further comprise fixing the cell stack 12 in the compressed state. This may comprise providing a single or multi-piece housing around the cell units 14 and connecting said housing to the end plates 18, 20, for example, by welding.

[0107] After fixing the cell stack 12, the cell stack 12 may be removed from the compression device 10. For this, the nuts 44 may be loosened such that the first compression plate 24 and the second compression plate 26 can be displaced away from each other along the compression axis 30.

[0108] In other embodiments, the compression device may remain with the cell stack 12, thus forming a cell stack assembly. In this case, the first compression plate 24 and the second compression plate 26 may form the end plates of the assembly.

[0109] In the following, a method of manufacturing a compressed cell stack using a compression device according to the second embodiment (Figure 2) will be described with reference to Fig. 6.

[0110] In a first step (see block 100 in Figure 6), a compression device 10 as described above is provided and a cell stack 12 is placed in the compression space 28 between the first compression plate 24 and the second compression plate 26 of the compression device 14.

[0111] For this, the first compression plate 24 may be held at distance from the second compression plate 26

[0112] (e.g. by a user). Alternatively, the first compression plate 24, the spring device 36 and the third compression plate 46 may initially be removed from the linear guides 34, and the cell stack 12 may be arranged on the second compression plate 26. The first compression plate 24, the spring device 36 and the third compression plate 46 may then be placed on the cell stack 12 such that the linear guides 34 extend through the first compression plate 24, the spring members 28, and the third compression plate 46.

[0113] In a next step, the cell stack 12 is compressed along the compression axis 30 (see block 102 in Fig. 6). For this, the third compression plate 46 is adjusted towards the second compression plate 26 along the compression axis 30 (guided by the linear guides 34), for example, by applying load to the third compression plate 46 using a mechanical press 60.

[0114] This compresses the spring members 28 located between the third compression plate 46 and the first compression plate 24, and thus loads them. Specifically, the third compression plate 46 is displaced towards the second compression plate 26 (and preferably towards the first compression plate 24) until the spring members 28 are compressed to a compression within the second section II of the characteristic curve 48 of the spring device (illustrated by the "X" in the exemplary force-compression curve 48 shown on the right in Figure 6, block 104).

[0115] After that, the clamping device 42 is activated (block 104 in Fig. 6), and the load applied to the third compression plate (e.g. via the press) is removed. In the specific example, activating the clamping device 42 comprises screwing the nuts 44 on the threaded portions of the linear guides 34.

[0116] Similar to the method described in connection with Figure 5, the method may further comprise stack conditioning processes. In the example of Figure 6, the method comprises a baking step (see block 106 in Figure 6). Specifically, the cell stack 12 and the compression device 10 are heat-treated in an oven 62 or hotbox, for example at temperatures between 300 °C and 700 °C.

[0117] As illustrated by the force-compression diagram depicted on the right of block 106 in Figure 6, during the baking step, shrinkage of the cell stack 12 and thus reduction in compression of the spring members 28 may occur, e.g. due to relaxation of the insulation plates 21 and / or the gaskets 22. Specifically, the spring device 36 is configured such that shrinkage occurring during the stack baking process is sufficient to change compression of the spring device 36 from the second section II to the first section I (illustrated by the "X"). Due to the relatively low gradient in the first section I, further shrinkage of the cell stack 12 (e.g. due to further relaxation of the gaskets 22 during further processing steps or during normal use of the cell stack 12) does not lead to significant change in spring force and thus compression force. As set out above, this allows to maintain a relatively constant compression force in the cell stack 12.

[0118] As discussed in connection with the method according to Figure 5, the method may further comprise fixing the cell stack 12 in the compressed state and, optionally, removing the cell stack 12 from the compression device 10.

[0119] In other embodiments, the compression device may remain with the cell stack 12, thus forming a cell stack assembly. In this case, after compressing the cell stack 12 along the compression axis 30 (see block 102 in Fig. 6), the third compression plate 46 and the second compression plate 26 may be held at a fixed distance from each other along the compression axis 30 such that the spring device 36 located between the third compression plate 46 and the first compression plate 24 is held in the deflected state. For this, the clamping device 42 may be activated as described above, and the load applied to the third compression plate (e.g. via the press) may be removed. In this case, the clamping device 42 may form part of the cell stack assembly. Alternatively or additionally, after compressing the cell stack 12 along the compression axis 30 (see block 102 in Fig. 6), a housing may be attached to the second compression plate 26 and the third compression plate 46, e.g. by welding, to maintain a spacing therebetween. In such embodiments, optional stack conditioning processes may be performed after applying the clamping device 42 or after applying the housing.

[0120] Figures 5 and 6 are described with reference to a non-liner force-compression curve similar to that of Figure 3. Other non-linear force-compression curves may be used, including ones similar to that of Figure 4.

Claims

Claims1. Compression device (10) for exerting a compressive force on an electrochemical cell stack (12), comprising:- a first compression plate (24) and a second compression plate (26), said first and second compression plates (24, 26) being displaceable towards each other along a compression axis (30) for compressing an electrochemical cell stack (12) received between them,- a spring device (36), comprising at least one spring member (38), for exerting a spring force on the first compression plate (24), said spring force acting along the compression axis (30), wherein: the spring device (36) is deflectable, preferably compressible, along the compression axis (30), the spring device (36) has a non-linear force-deflection, preferably force-compression, behaviour in that a characteristic curve (48) representing the progression of the spring force exerted by the spring device (36) as a function of the deflection, preferably compression, of the spring device (36) along the compression axis (30) has a first section (I) having a first average gradient (50) and a second section (II) at higher deflection having a second average gradient (52), said second average gradient (52) being larger than said first average gradient (50).

2. The compression device (10) according to claim 1, wherein the second average gradient (52) is at least two times larger than the first average gradient (50), preferably at least three times, more preferably at least five times.

3. The compression device (10) according to claim 1 or 2, wherein the first section (I) extends over a first deflection range (54) and the second section (II) extends over a second deflection range (56), wherein the first deflection range (54) is at least two times larger than the second deflection range (56), preferably at least three time larger, more preferably at least five times larger.

4. The compression device (10) according to any one of the preceding claims, wherein the first section (I) extends over a deflection range (54) of at least 2.0 mm, preferably at least 3.0 mm.

5. The compression device (10) according to any one of the preceding claims, wherein the second section (II) extends over a deflection range (56) of at most 3.0 mm, preferably at most 2.0 mm, more preferably at most 1.0 mm, more preferably at most of 0.5 mm.

6. The compression device (10) according to any one of the preceding claims, wherein an average spring force in the second section (II) is between 2 and 8 times greater than in the first section (I), preferably between 3 and 6 times greater.

7. The compression device (10) according to any one of the preceding claims, wherein the spring device (36) has a progressive characteristic in the second section (II).

8. The compression device (10) according to any one of the preceding claims, wherein the first section (I) is a plateau section, said plateau section being characterised in that within said plateau section, the spring force changes by at most 10%, preferably by at most 5%.

9. The compression device according to any one of the preceding claims, wherein in the first section (I) the spring force changes by at most 10%, preferably by at most 5%, around a target spring force, said target spring force being between 1 and 100 kN, preferably between 1 and 50 kN.

10. The compression device (10) according to any one of the preceding claims, wherein the characteristic curve (48) has a third section (III) at lower deflection than the first section (I), preferably adjoining the first section (I) at a low deflection value of the first section (I).

11. The compression device (10) according to the preceding claim, said third section (III) having a third average gradient (58) that is larger than the first average gradient (52).

12. The compression device according to claim 10 or 11, wherein the spring device (36) has a digressive characteristic in the third section (III).

13. The compression device (10) according to any one of the preceding claims, wherein the or each spring member (38) comprises at least one disc spring.

14. The compression device (10) according to any one of the preceding claims, further comprising a third compression plate (46) located to a side of the first compression plate (24) opposite to the second compression plate (26).

15. The compression device (10) according to the preceding claim, wherein the spring device (36), preferably the at least one spring member (38), is interposed between the first compression plate (24) and the third compression plate (46).

16. The compression device (10) according to any one of the preceding claims, further comprising a clamping device (42), said clamping device (42) being configured to deflect, preferably compress, the spring device (36) along the compression axis (30) and / or to hold the spring device (36) in the deflected, preferably compressed, state.

17. The compression device (10) according to the preceding claim when referring to claim 14, wherein the clamping device (42) is configured to limit axial displacement of the third compression plate (46) along the compression axis (30) in a direction away from the second compression plate (26).

18. The compression device (10) according to any one of the preceding claims, further comprising a linear guide device (32) for guiding an axial displacement of the first compression plate (24), and preferably the at least one spring member (38), along the compression axis (30).

19. The compression device (10) according to the preceding claim, wherein the linear guide device (32) comprises one or more linear guides (34), preferably one or more guiding rods, said linear guides (34) extending along the compression axis (30).

20. The compression device (10) according to the preceding claim, wherein the or each spring member (38) of the spring device (36) surrounds an associated linear guide (34).

21. The compression device (10) according to any one of claims 18 to 20 when referring to claim 16, wherein each linear guide (34) comprises a threaded portion, and wherein the clamping device (42) comprises at least one nut (44) or clamp configured to cooperate with said threaded portion to deflect, preferably compress, the spring device (36), preferably the at least one spring member (38), along the compression axis (30) and / or to hold the spring device (36), preferably the at least one spring member (38), in the deflected, preferably compressed, state.

22. Cell stack assembly, comprising an electrochemical cell stack (12), comprising a plurality of cell units (14) stacked upon one another along a stacking direction (16), and- a compression device (10) according to any one of the preceding claims, wherein said electrochemical cell stack (12) is held in a compressed state between the first compression plate (24) and the second compression plate (26) of the compression device (10).

23. Method of manufacturing an electrochemical cell stack (12), comprising:- providing a compression device (10) according to any one of the preceding claims;- providing an electrochemical cell stack (12) between the first compression plate (24) and the second compression plate (26) of said compression device (10);- adjusting said first and second compression plates (24, 26) towards each other along the compression axis (30) to compress the electrochemical cell stack (12), preferably until a preset compression grade of the cell stack (12) is reached;- deflecting, preferably compressing, the spring device (36), preferably the at least one spring member (38), to a deflection, preferably compression, within the second section (II) of the characteristic curve (48) of the spring device (36), and, preferably, holding the spring device (36) in said deflected, preferably compressed, state.

24. Method according to the preceding claim, wherein adjusting the first and the second compression plates (24, 26) towards each other along the compression axis (30) comprises applying load to the first plate (24), preferably using a press.

25. Method according to claim 23 or 24, wherein the compression device (10) comprises a clamping device (42) configured to deflect, preferably compress, the spring device (36) along the compression axis (30) and to hold the spring device (36) in the deflected, preferably compressed, state, wherein deflecting, preferably compressing, the spring device (36) is performed using said clamping device (42).

26. Method of manufacturing an electrochemical cell stack (12), comprising:- providing a compression device (10) according to claim 15, said compression device (10) comprising a clamping device (42) configured to limit axial displacement of the third compression plate (46) along the compression axis (30) in a direction away from the second compression plate (26);- providing an electrochemical cell stack (12) between the first compression plate (24) and the second compression plate (26) of said compression device (10);- adjusting said first and second compression plates (24, 26) towards each other along the compression axis (30) to compress the electrochemical cell stack (12) by applying load to the third compression plate (26) along the compression axis (30), preferably using a press, such that the spring device (36), preferably the at least one spring member (38), located between the first compression plate (24) and the third compression plate (46) is deflected, preferably compressed, to a deflection, preferably compression, within the second section (II) of the characteristic curve (48) of the spring device (36);- activating the clamping device (42) of the compression device (10) to block axial displacement of the third compression plate (46) and the second compression plate (26) away from each other along the compression axis (30).

27. Method according to any one of claims 23 to 26, further comprising hot conditioning or baking the electrochemical cell stack (12) and the compression device (10).

28. Method according to any one of claims 23 to 27, further comprising fixing the electrochemical cell stack (12) in the compressed state, preferably by providing a housing around the electrochemical cell stack (12).

29. Method according to any one of claims 23 to 28, further comprising: removing the cell stack (12) from the compression device (10), preferably including deactivating the clamping device (42).

30. Method according to any one of claims 23 to 29, wherein the electrochemical cell stack (12) comprises at least one gasket (22).

31. Method according to any one of claims 23 to 30, wherein the electrochemical cell stack (12) comprises at least one electrically insulating plate (21).

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