Power connection for electrochemical cell stack

Abstract

An electrochemical cell assembly 10 comprises a stack of cell units 12, at least one positive and at least one negative electrical end plate 14a-b, and a top plate 16; wherein at least one of the elec

Description

Field of the Invention The present invention relates to electrochemical cell stacks, in particular, fuel cell stacks and electrolyser cell stacks, and to the design of their electrical end plates and electrical terminals. The cell stacks of the present invention include cells of solid oxide, polymer electrolyte membrane, and molten carbonate types. The present invention more specifically relates to solid oxide fuel cell (SOFC) and solid oxide electrolyser cell (SOEC) stacks, and these may include metal-supported solid oxide fuel cell (MS-SOFC) or electrolyser cell stacks (MS-SOEC). Background to the Invention Some fuel cell units can produce electricity by using an electrochemical conversion process that oxidises fuel to produce electricity. Some fuel cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as electrolyser fuel cell units, for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide. They may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-400 fuel cell units in a stack, with the individual fuel cell units arranged electrically in series. A solid oxide fuel cell (SOFC) that produces electricity is based upon a solid oxide electrolyte that conducts negative oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the anode (fuel electrode) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (air electrode). Conventional ceramic-supported (e.g. anode-supported) SOFCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs have recently been developed which have the active fuel cell component layer supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOFC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOFCs and can be manufactured using conventional metal welding techniques. A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC but is essentially that SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide by input of electrical energy and using the solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen. The present invention is directed at a stack of repeating electrochemical cell units and concerns the design of their electrical end plates (power take-off or delivery). It is thus applicable to various types of fuel and electrolyser cells, for example, based on solid oxide electrolytes, polymer electrolyte membranes, or molten electrolytes. For convenience, "cell units" is used to refer to "electrochemical cell units" including fuel or electrolyser cell units. The electrical energy produced by a fuel cell (or input to an electrolyser cell) may be transferred through a stack of cell units and transferred from (or to) the stack using at least two electrical studs (of opposite electrical polarity) and associated electrical end plates which make electrical contact between the studs and the ends of the stack. The electrical studs and electrical end plates may also be referred to as positive and negative "power take offs" or "electrical terminals", which terminology is used for convenience regardless of whether the power is being taken off (as in case of fuel cell) or delivered (as in case of electrolyser cell). The stack is typically enclosed in a vessel to form a fluid volume and thereby to retain one of the fluids (fuel or air and / or exhaust gasses) for use in - or exhaust from - the stack. The electrical studs typically pass through the vessel in order to allow electrical energy to be transferred between the stack and a load or source external to the stack (the electrical stud, or bolt, passed through an opening in the vessel for external connection, the (distal) portion of the stud external to the vessel may form a terminal). A fluid seal is typically required to be maintained between the power take offs and the vessel in order to maintain the integrity of the fluid volume enclosed by the vessel. A compressive stress applied to a gasket type material is a typical method of achieving such a seal. Operating a fuel cell (for example an SOFC) system where the cell stack operates in the 450-650°C range (for example, intermediate-temperature solid oxide fuel cell IT-SOFC) results in a challenging set of technical problems being encountered in transferring the electrical energy to / from the stack while maintaining a fluid seal. WO2022 / 043085 Al relates to a sealing mechanism for a SOFC or SOEC. It discloses a stack of cells held in compression between a top plate and a base plate. Positive and negative terminals are provided on the top plate, which are electrically connected to a stud connected through an opening in the top plate to a corresponding electrical end plate held in compression between the top plate and the base plate. A fluidic seal is maintained by the compression means between the base portion and the respective one of the base plate and top plate, to prevent loss of fluid through the opening. However, over the life of the stack the compressive forces maintaining the fluidic seal reduce. The inventors of the present invention have discovered this is predominantly due to material creep. This loss of compression can lead to fluid leaking, which leads to a reduction in performance and potentially an increased safety risk. The present invention seeks to address, overcome or mitigate at least one of the prior art disadvantages. According to a first aspect there is provided an electrochemical cell assembly according to claim 1. The electrochemical cell assembly comprises a top plate between positioned on top of a stack of cell units and at least one positive and at least one negative electrical end plate. This stack is preferably disposed in compression by means of compression means acting between a base plate and top plate. Compression provides sealing between the different fluids, and also improves the electrical connection between cell units. At least one of the electrical end plates is connected or integrally formed with, and in electrical contact with, an electrical stud that extends from the at least one electrical end plate and passes through an opening in the top plate to form an electrical terminal. Sealing means is provided to prevent loss of fluid through the opening. Preferably, the sealing means being disposed in compression by the compression means. The sealing means may comprise gaskets or the like; detail regarding suitable sealing means can be found in WO2022 / 043085 Al. Biasing means is provided at the electrical stud to bias the electrical end plate away from the stack of cell units (in the stacking direction) and against the top plate. The biasing means provides a secondary compression force to the sealing which can complement the compression or compensate for a relaxation in the compression provided between the base plates. This a benefit especially for pressurised electrochemical cell assemblies where an increase in internal pressure after manufacturing reduces sealing efficacy. The electrochemical cell assembly may be a fuel cell (converting a fuel to energy), or an electrolyser (decomposing a compound into constituent parts using electricity). The fuel cell or electrolyser cell may be solid oxide based, termed SOFC for fuel mode and SOEC for electrolysis mode, or SOC in general. SOC operate at elevated temperatures (400-800 °C) and as such material creep can be significant. Optionally, each of the cell units is provided with at least one port and the cell units are stacked one upon another such that the respective ports align to form a respective internal manifold extending through the stack. The electrical stud extending through the respective opening is also in alignment with the respective internal manifold such that the compression forces exerted by the compression means act to seal the respective internal manifold and act to seal the opening. In such a way, the biasing force acts to improve sealing at the ports independently of the compression means. Optionally, the biasing means consists of a tensioning means to exert a force between the electrical terminal and the top plate. Optionally the biasing means in a state of compression. Optionally the biasing means comprises a spring element such as a coil spring, spiral spring, flat spring. Such an arrangement is simple and can be manufactured to withstand high temperatures. Optionally, the biasing means comprises one or more conical spring washers. Such an arrangement provides a degree of adjustment, for example by selecting an appropriate number or orientation of washers to achieve a desired biasing force. Optionally, the biasing means is formed of a resilient material. For example, achieved by compression of a material that will resist compressive forces. Optionally, the biasing means is formed of a mechanical member in flexion. Optionally, the biasing means is formed of a screw mechanism on the electrical stud. Such an arrangement allows for adjustment to the biasing force. Optionally, the biasing means is formed of a rotary spring connected to a screw mechanism on the stud. Optionally the biasing means comprises a material with a higher coefficient of thermal expansion than the electrical stud. This provides an axial force on the electrical stud, providing an increased gasket compression force at the operating temperature and reducing leak rate. Optionally, the electrochemical cell assembly further comprises a gasket positioned around the electrical stud between the top plate and the electrical end plate and means for retaining the gasket. The means for retaining the gasket may comprise a protrusion, widening of the stud or other feature to retain the gasket in place. Optionally the biasing means is provided by a pressure differential across the sealing means. In one example, the pressure differential is applied on either side of a gasket positioned around the electrical stud. In use, the pressure differential forces the gasket towards the exterior of the stack, improving sealing performance. Optionally, the electrochemical cell assembly further comprises an electrical terminal tab extending in a direction perpendicular to the stacking direction. This allows for convenient electrical connection without interfering with the biasing means. Optionally, the electrical terminal tab is affixed to the top plate at a location offset from the electrical stud. Such an arrangement provides rigidity to the electrical terminal so that tortional forces are not transferred to the electrical end plate, for example when connecting or disconnecting the electrical connection. Optionally, the electrical terminal tab comprises a deflection in the stacking direction between the electrical terminal and the affixing point on the top plate. Such an arrangement compensates for the additional height of the biasing means for a more compact assembly. In another aspect, there is provided a method of manufacturing an electrochemical cell assembly, the method comprising stacking a plurality of cell units, at least one positive and at least one negative electrical end plate between a base plate and a top plate. The electrical end plate is connected (or integrally formed with) and in electrical contact with, an electrical stud that extends from a base portion of the electrical end plate and passes through an opening in the top plate to form an electrical terminal. This provides access to form an electrical terminal. Sealing means, preferably in the form of gaskets or the like, is provided at the opening in the top plate to reduce or eliminate leaks through this opening. The stack is then compressed; and biasing means are provided at the electrical stud, to bias the electrical end plate towards the top plate. The biasing means provides an additional sealing mechanism independent of the stack compression. As indicated above, "cell units" or "cell stack" is used to refer to "electrochemical cell units" or "electrochemical cell stack". Brief Description of the Drawings Figure 1 shows a cross section of a portion of an electrochemical cell assembly showing a first example of a power connection; Figure 2a shows a cross section of a portion of an electrochemical cell assembly showing a second example of a power connection; Figure 2b shows an alternative arrangement of conical washers to that shown in Fig. 2a; Figure 3 shows a cross section of a portion of an electrochemical cell assembly showing a third example of a power connection; and Fig 4a shows a cross section of a portion of an electrochemical cell assembly showing a power connection in a different location; Fig 4b shows an expanded view of the electrical stud shown in Fig 4a Fig 5 is a flow diagram showing a simplified method of assembling an electrochemical cell assembly. Detailed Description Figure 1 shows a cross section of a portion of an electrochemical cell assembly 10. This shows a stack of cells 12 (also referred to as 'repeat units') which each contain an electrochemically active portion deposited on a (preferably metal) substrate 6. A metal interconnect 8 is welded or otherwise affixed to the opposing side of the metal substrate 6 to the electrochemically active portion. This interconnect contacts the electrochemically active portion of a neighboring cell to connect the cells electrically in series. The fuel side (steam / hydrogen enriched steam in electrolysis mode, or hydrogen-containing gas in fuel cell mode) is kept separated from the off gas (oxygen enriched air, or air) by gaskets 9. These gaskets 9 are maintained in compression to reduce leaks, this compression also acts to improve electrical connection via the interconnects 8. An electrical end plate 14a is positioned over the stack of cells 12 and is in electrical contact with an upper-most cell. The cells 12 and the electrical end plate 14a are stacked between a top plate 16 and a base plate 15. Compression means acts between the top and base plates to seal fluid paths between the cells and to improve electrical conductivity. In one example, this is provided by compressing the stack of cells 12 between the top and base plates and then welding a metal 'skirt' 4 to retain the stack under compression. An electrical stud extends 18 from the electrical end plate 14a and out of the top plate 16. This is to allow an external electrical connection to provide power (in fuel cell mode) or to receive power (in electrolysis mode). Electrical insulation 20a is provided between the electrical end plate 14a and the top plate 16 and around the electrical stud 18. Fluid sealing 21 is provided at the aperture of the end plate 15. At the other end of the stack, an opposingly charged electrical endplate 14b provides an opposing terminal (not shown). Such a terminal may be provided at the same end of the stack as the electrical stud 18 via a busbar, or using a similar arrangement as at the top of the stack with a stud extending through the baseplate 15. Electrical insulation 20b is provided between the electrical end plate 14b and the baseplate 15. Biasing means 22 is provided at the electrical stud 18 to bias the electrical end plate 14 towards the end plate 16. In the example shown in Figure 1, this is a spring in a state of compression positioned around electrical stud 18. The spring is inhibited between the end plate 16 and the top of the electrical stud 18. In such a way, the biasing means acts to 'pull' the electrical end plate 14 upwards, towards the top plate 16. This provides an additional compression means which improves the sealing at the interface between the top of the stack of cells 12 and the top plate 16. This additional compression mechanism compensates for loss in stack compression. This may occur due to material creep, or the insulation layer(s) 20 shrinking, during the lifetime of the electrochemical cell assembly. The end of the electrical stud 18 in Figure 1 is shown to be coupled to an electrical terminal tab 24. However, a bolt, cap, or protrusion would serve a similar purpose. Similarly, one end of the spring could be welded to the electrical stud 18 to inhibit its movement. The electrical terminal tab 24 extends away from the electrical stud 18 in a direction perpendicular to the stacking direction. This allows an electrical connection to be made without interfering with the electrical stud 18 which may otherwise adversely affect sealing performance. The electrical terminal tab 24 is affixed to the top plate 16 at a location offset from the electrical stud 18. When connecting an electrical connection to the electrical terminal tab 24, twisting or pulling forces may be applied. The connection to the top plate 16 inhibits these forces being transferred to the electrical stud 18 and thus the electrical end plate 14a. Figure 2a shows a similar view to that of Figure 1, but instead of a spring, the biasing means 22 is a stack of conical washers (more specifically, frustoconical). When compressed, a conical washer exerts a restoring force, thus acting like a spring. The spring constant can be modified by stacking a number of washers together, and further modified by the orientation of stacking, i.e. placing a conical on top of another either in the same orientation, or in opposing orientations. Figure 2b shows an alternative arrangement of washers which would have a different spring constant to that shown in Figure 2a. Other possible biasing means include a resilient material which, when under compression, elastically deforms and exerts a restoring force. Suitable materials may include rubber or polymer. The biasing means may alternatively be a mechanical member in flexion as is shown in Figure 3. In such an example, a metal plate or rod may be affixed to the top plate 16 and also at or near the electrical stud 18. During assembly, the plate or rod is 'bent' so that it is under flexion and as such exerts a force upwards on the electrical stud 18. Figure 3 shows an example where the electrical tab 24 performs the function of the biasing means 22 by being affixed to the electrical stud 18 and the top plate 16 in flexion held in place by a cap 25. In a further example, the biasing means may also include a screw mechanism on the electrical stud 18. This allows for the compression to be adjusted, either upon assembly, or during use. For example, the cap 25 may be tightened to increase the force imparted by the mechanical member in flexion. Alternatively, the screw mechanism may be coupled with a rotary spring which is wound as the screw mechanism is tightened. The biasing means may alternatively be an element positioned on / around the electrical stud which has a higher coefficient of thermal expansion than the electrical stud so that it expands when at an (elevated) operating temperature. This expansion would push against the top plate 16 in a similar manner to the other biasing means described above. Figure 4a shows an alternative embodiment where the electrical stud 18 is in a different location. Rather than being in line with the ports, the electrical stud is to one side of the cells 12. The electrical stud 18 passes through the top plate 16 and sealing means 21 is provided in a similar manner to earlier examples. A key difference is that a gasket 26 is positioned around the electrical stud 18 to improve the sealing. As shown in the expanded view of Fig 4b, the gasket 26 is supported by a protrusion 28. The protrusion 28 provides a sealing surface and enables the biasing means 22 to bias the gasket 26 against the top plate 16. In such a way, the sealing performance is improved independently of the compression applied to the stack. The gasket 26 is preferably be made of a resilient material so that it can adapt to potentially changing compressive forces - for example due to changing temperatures. A vermiculite-based material has been found to be appropriate. The biasing means 18 is shown on the outside of the top plate 16, but it may be within the stack 10. For example, the biasing means may comprise an element on the electrical stud which has a higher coefficient of thermal expansion than the electrical stud so that it expands when at an (elevated) operating temperature. Such an element may be positioned beneath the gasket 26 and restrained so that when expanded it exerts a force on the gasket 26 toward the top plate 16. This performs the same role as a biasing means external to the stack 10 as depicted in Figure 4. A similar force can be provided at the gasket 26 by a pressure differential between the interior and exterior of the stack 10. If the interior of the stack 10 is at a higher pressure than the exterior, a force will be exerted upwards on the electrical end plate 14a, pushing the electrical stud 18 (and electrical end plate 14a) towards the top plate 16. Such a force improves the sealing at the gasket 16, but also at other locations around the electrical end plate 14a. In such a way, the protrusion 28 retaining the gasket 26 may be considered a biasing means 18a. It should be appreciated that the features described with reference to Fig 4 could be applied to the arrangements shown in Figures 1-3 where the electrical stud is positioned in-line with the ports. In such an example, the biasing means 22 would act to improve sealing at the electrical end plate 14a, but also at the top plate 16. In any of the examples described above, a combination of biasing means may be provided, for example a spring with resilient members at either end, or an electrical tab 24 in flexion with stack of conical washers. A further difference over Figure 1 shown in both Figures 2a, 3 and 4 is that the electrical terminal tab 24 is bent between the electrical stud 18 and where it is affixed to the top plate 16. The bend in a direction parallel to the stacking direction. In Figure 2a it is a discrete bend and is a curve in Figure 3. This bend provides a reduced overall stack height when an electrical connection is made. This bend in the electrical terminal tab can be applied independently of choice of biasing means 22, for example, it could equally apply to the spring of Fig. 1. Both Figures 2a, 3 and 4 show the electrical terminal tab 24 bending back to being perpendicular to the stacking direction, but this may be avoided by a different fixing mechanism to the top plate 14. Figure 5 shows a simplified flow diagram of a method of assembling an electrochemical cell assembly incorporating a biasing means as shown in Figures 1 to 3. The first step SI is to stack the cell units (typically at least 300 cell units), together with at least one positive and at least one negative electrical end plate between a base plate and a top plate. Gaskets are provided between cell units so as to provide a seal between fuel and oxidant sides of the cell units. Other components such as electrical isolation, electrical connections, thermal insulation, and fluid guiding / blocking features may also be provided within the stack. As discussed with reference to Figures 1 and 2, at least one of the electrical end plates is connected or integrally formed with, and in electrical contact with, an electrical stud which passes through an opening in the top plate to form an electrical terminal. Sealing means is provided at the opening in the top plate during or after the stacking process. The next step S2 is to compress the stack. This may be achieved by a mechanical compression which exerts a specified force on the top plate, pushing it towards the base plate. The force exerted would depend on the materials used in the stack, in particular the gaskets, to ensure appropriate leak sealing. 5 The compression is held, preferably by welding a skirt to the top and base plates. Tie bars could be used as an alternative (or in addition to) a skirt to maintain stack compression. Biasing means is provided at the electrical stud in step S3, to bias the electrical end plate towards the top plate. This step could form part of a step of assembling the electrical connections. In one example, a biasing means is placed over the stud and then held in compression while an electrical tab is welded 10 onto the electrical stud. It should be appreciated that steps S2 and S3 may be performed in the reverse order, or simultaneously. The present invention is not limited to the above examples only, and other examples will be readily apparent to one of ordinary skill in the art without departing from the scope of the appended claims. 15 For example, the terms 'base' and 'top' could be reversed so that an electrical stud protrudes from the base of the stack as opposed to the top. Similarly, electrical studs could protrude from both the base and the top of the stack.

Claims

1. An electrochemical cell assembly comprising:a stack of cell units,at least one positive and at least one negative electrical end plate, anda top plate positioned on top of the stack of cell units and the at least one positive and at least one negative electrical end plate;wherein at least one of the electrical end plates is connected or integrally formed with, and in electrical contact with, an electrical stud that extends from the electrical end plate and passes through an opening in the top plate to form an electrical terminal;sealing means at the opening in the top plate; andbiasing means, provided at the electrical stud, to bias the electrical stud away from the stack of cell units in the stacking direction.

2. An electrochemical cell assembly according to any preceding claim wherein the means to bias the electrical end plate against the top plate consists of a tensioning means to exert a force between the electrical terminal and the top plate3. An electrochemical cell assembly according to any preceding claim wherein the biasing means in a state of compression.

4. An electrochemical cell assembly according to any preceding claim wherein the biasing means comprises a spring element such as a coil spring, spiral spring, flat spring.

5. An electrochemical cell assembly according to any preceding claim wherein the biasing means comprises one or more conical spring washers.

6. An electrochemical cell assembly according to any preceding claim wherein the biasing means is formed of a resilient material.

7. An electrochemical cell assembly according to any preceding claim wherein the biasing means is formed of a mechanical member in flexion.

8. An electrochemical cell assembly according to any preceding claim wherein the biasing means comprises a screw mechanism on the electrical stud.

9. An electrochemical cell assembly according to the preceding claim wherein the biasing means comprises a rotary spring connected to a screw mechanism on the stud10. An electrochemical cell assembly according to any preceding claim further comprising: a gasket positioned around the electrical stud between the top plate and the electrical end plate, andmeans for retaining the gasket.

11. An electrochemical cell assembly according to the preceding claim wherein the biasing means comprises a material with a higher coefficient of thermal expansion than the electrical stud.

12. An electrochemical cell assembly according to any preceding claim further comprising a base plate and wherein the stack of cell units and the at least one positive and at least one negative electrical end plate are disposed in compression by means of compression means acting between the base plate and top plate;the sealing means being disposed in compression by the compression means; and the biasing means being adapted to bias the electrical end plate towards the top plate.

13. An electrochemical cell assembly according to the preceding claim wherein each of the cell units is provided with at least one port and the cell units are stacked one upon another such that the respective ports align to form a respective internal manifold extending through the stack, and,the electrical stud extending through the respective opening is also in alignment with the respective internal manifold,such that the compression forces exerted by the compression means act to seal the respective internal manifold and act to seal the opening.

14. An electrochemical cell assembly according to any preceding claim comprising an electrical terminal tab extending in a direction perpendicular to the stacking direction.

15. An electrochemical cell assembly according to claim 14 wherein the electrical terminal tab is affixed to the top plate at a location offset from the electrical stud.

16. An electrochemical cell assembly according to claim 15 wherein the electrical terminal tab comprises a deflection in the stacking direction between the electrical terminal and the affixing point on the top plate.

17. A method of assembling an electrochemical cell assembly, the method comprising:stacking a plurality of cell units, at least one positive and at least one negative electrical end plate between a base plate and a top plate;wherein at least one of the electrical end plates is connected or integrally formed with, and in electrical contact with, an electrical stud that extends from a base portion of the at least one electrical end plate and passes through an opening in the top plate to form an electrical terminal;providing sealing means at the opening in the top plate;compressing the stack; andproviding biasing means at the electrical stud, to bias the electrical end plate towards the top plate.

18. A method according to claim 17 comprising compressing the biasing means when compressing the stack.

19. A method according to claim 17 or 18 further comprising affixing a skirt to the top and bottom plate to retain the compression.Application No: GB2411483.7 Examiner: Vivek VijayaraghavanClaims searched: 1-19Date of search: 30 January 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-19 US 10693173 B2 (KAKUWA) See figure 1 and 3 X 1-19 WO 2022 / 043085 Al (CERES IP CO LTD) See figure 5B especially X 1-19 US 5419981 A (GOLBEN) See figure 1 v A 1-19 US 2005 / 0186462 Al (BELANGER et al.) See figure 2-6 X 1-19 WO 2023 / 117088 Al (CERES IP CO LTD) See figure 1Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if p Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP. WO &US patent documents classified in the following areas of the UKCX :Worldwide search of patent documents classified in the following areas of the IPC____________C25B; H01M_______________________________________________The following online and other databases have been used in the preparation of this search reportSEARCH-PATENTInternational Classification:Subclass Subgroup Valid From HO IM 0008 / 2485 01 / 01 / 2016 C25B 0009 / 60 01 / 01 / 2021 C25B 0009 / 73 01 / 01 / 2021 HO IM 0008 / 248 01 / 01 / 2016

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