ELASTOMERIC END FRAME OF A REDOX FLOW BATTERY

Description

[0001] The present invention relates to an elastomeric end frame of a redox flow battery with a central first recess for receiving a current collector and a radially outer frame with a first end face surrounding the central recess, and a cell stack with such an end frame.

[0002] Redox flow batteries are known to consist of cells through which electrically differently charged electrolytes flow. The cells comprise two frames arranged in series, each containing an electrode separated by a semipermeable membrane, typically an ion exchange membrane. A bipolar plate is positioned between individual cells of the redox flow battery. However, the end frames at the axial ends of the redox flow battery do not contain an electrode as in the cells, but rather a metallic current collector connected to an external electrical terminal of the redox flow battery.The individual frames of the redox flow battery must be sealed against each other to prevent electrolyte from leaking out (external sealing) and to prevent the differently charged electrolytes from mixing (internal sealing), which would result in a loss of efficiency. However, leaks in the area of ​​the metallic current collector are problematic, as electrolyte on the collector would inevitably cause corrosion, eventually leading to the complete failure of the redox flow battery. Therefore, contact between the electrolyte and the metallic current collector must be prevented.

[0003] Redox flow batteries utilize two types of frames: those made of elastic plastics (elastomers) and those made of non-elastic plastics, such as PVC, PP, PE, PTFE, epoxy resin, etc. Non-elastic plastics, typically thermoplastics, are rigid and only malleable within a certain temperature range. Elastomers are dimensionally stable but elastically deformable plastics, meaning that an elastomer returns to its original shape after deformation.

[0004] Sealing elements, such as O-rings, are typically used to create a seal between rigid frames made of non-elastic plastics. These elements are positioned between the rigid frames and seal by being compressed. The sealing elements are usually arranged in grooves on the end faces of the frames. Examples can be found in EP 1 411 576 A1 or WO 2004 / 079849 A1. The problem is that the rigid frames can be relatively easily damaged mechanically, for example, by scratches on the surface. Experience has shown that even small scratches are sufficient to permanently impair the sealing effect. Such seals are also sensitive to foreign matter (hair, fibers, etc.) on the frames, which, in combination with the sealing elements, can again lead to leaks.In addition, the assembly of a redox flow battery is made more difficult by the large number of necessary sealing elements, which can easily slip and get pinched.

[0005] Frames made of non-elastic plastics are also known that have raised, circumferential ribs molded onto one end face. These ribs engage in grooves on the adjacent end face to create a form-fitting seal. Such frames are known, for example, from US Patents 4,640,876 A and 6,086,643 A. These frames are also highly susceptible to even the slightest surface damage, which can compromise their sealing performance. Another problem with such frames is the manufacturing tolerances, typically achieved through injection molding. Manufacturing tolerances of + / -1% are common. This can easily lead to misalignment between the ribs and their corresponding grooves, especially when multiple ribs are used, which in turn can result in leaks between the frames.

[0006] For elastomeric stack frames with molded raised sealing elements, such as sealing ribs, two differently designed stack frames would be needed to prevent the sealing elements from touching each other in the assembled state. This would, in turn, lead to problems with the precise alignment of the sealing elements. With sealing elements lying side by side, the deformation of the sealing elements during assembly would also create the problem of potentially overlapping sealing elements. Furthermore, even with elastic stack frames, problems with foreign matter (hair, fibers, etc.) on the sealing surfaces or minor damage (scratches) would arise. Therefore, the stack frames would have to be very carefully inspected for damage or foreign matter during assembly, which would significantly increase the assembly effort. All these problems would also lead to a higher susceptibility to leakage in a cell stack.Furthermore, this would also increase the complexity of assembling a redox flow battery, as more different individual parts would be required, ultimately increasing the potential for assembly errors. For these reasons, elastomeric frames with molded, raised sealing elements are generally not used.

[0007] For these reasons, elastomeric frames utilize the properties of the elastomeric material to create a seal by pressing the flat end faces of the adjacent stack frames together, thereby creating a seal between them. Such stack frames are known, for example, from AT 501 902 A1, US 2012 / 0156535 A1, and AT 501 903 A1. The advantage here is that the seal is created by the adjacent end faces themselves, eliminating the need for additional sealing elements or raised ribs. Furthermore, such an elastomeric stack frame with sealing end faces is less susceptible to surface damage. However, manufacturing tolerances in the production of these frames also present challenges.The redox flow battery requires a seal between two stack frames: one to the electrode plate for internal sealing and the other to the adjacent stack frame for external sealing. However, due to manufacturing tolerances, simultaneous sealing of both surfaces is not possible. The dimensions of the stack frame allow for prioritizing either the internal or external sealing effect by adjusting the frame's dimensions to increase the contact pressure between the electrode plate and the stack frame, or between the two stack frames. Generally, external sealing is preferred, as a small amount of internal leakage between the stack frame and the electrode plate is not a significant problem. However, this is not possible between the stack frame and the end frame of the redox flow battery, as such internal leakage of electrolyte fluid would corrode and destroy the metallic current collector in the end frame.

[0008] It is therefore an object of the present invention to create a seal between the end frame and the adjacent frame of a cell stack of a redox flow battery that seals securely both internally and externally.

[0009] This problem is solved by molding a first sealing element, extending along the circumference of the end frame and rising from the first end face, on the side of the recess for receiving a current collector. This design accepts the disadvantages of increased assembly effort (due to the need for a thorough inspection of the end frame for surface damage or foreign matter), as the sealing element creates a seal that reduces the likelihood of electrolyte fluid coming into contact with the current collector. External sealing is achieved as before by the adjacent end faces of the elastic end frames and the stack frame.

[0010] If the end frame has a circumferential protrusion with a second end face on its radially outer edge, and a second sealing element extending along the circumference of the end frame and rising from this second end face is integrally formed, the external sealing effect can be further enhanced. Furthermore, the sealing effect is no longer, or only minimally, affected by manufacturing tolerances of the end frame or stack frame.

[0011] By providing several sealing elements on the first end face or on the second end face, the sealing effect can be improved even further.

[0012] The present invention is described below with reference to the Figuren 1 bis 6 In more detail, the invention is explained, and exemplary, schematic, and non-restrictive embodiments are shown. This includes showing Fig.1 a redox flow battery with a cell stack, Fig.2 a section through the cell stack, Fig.3 und 4 Detailed views of the end frame and the adjacent stack frame, Fig.5 a perspective view of an end frame according to the invention and Fig.6 Possible cross-sections of the sealing elements on the end frame.

[0013] With reference to Figs.1 und 2 The structure of a redox flow battery 1 is described. A cell stack 4 of a redox flow battery 1 comprises a plurality of cells 2, each of which is formed from two stack frames 3. A stack frame 3 is made of an elastomer, such as a polyolefinic thermoplastic elastomer (TPE or TPO), such as Santoprene®, or a thermoplastic vulcanize (TPV), preferably by injection molding. The frame material has, for example, a hardness in the range of 40–95 Shore A, preferably 60–75 Shore A. A semipermeable membrane 7, typically an ion exchange membrane (either a cation or anion exchange membrane, e.g., Nafion®), is arranged between each pair of stack frames 3 of a cell 2. An electrode plate 5, e.g. a bipolar plate, is arranged between two adjacent cells 2, wherein the electrode plate 5 can be inserted into recesses in the stack frame 3, as shown here.The stack frames 3 have central, continuous recesses in which electrodes 6, e.g., carbon fiber mats, are arranged. Electrolyte fluids with different electrical charges are pumped through the cells 2 via bores 8, 9 in the stack frames 3, with the electrodes 6 of each half-cell of a cell 2 being supplied with a different electrolyte fluid. The electrolyte fluids can be supplied and removed externally via electrolyte fluid connections 10 and are then distributed internally via a channel system. As is well known, this results in the generation of electric current or the charging of the redox flow battery 1, or more precisely, the electrolyte fluids, through electrochemical processes.

[0014] The sealing between the stack frames 3 is achieved via the adjacent end faces of the stack frames 3. Since the stack frames 3 are made of an elastomer and are therefore elastic, sufficient pressure ensures a tight seal between the stack frames 3 or cells 2.

[0015] The cell stack 4 is arranged between two rigid end plates 11 and pressed together by clamping devices, such as through bolts 12, which are tightened by means of nuts 13, washers 15, and springs 14. An electrical connection 16 can also be provided on the end plates 11, via which the redox flow battery 1 can be connected to an external circuit. Furthermore, connections 10 for the supply and discharge of the electrolyte fluid are provided on the end plates 11. The two end plates 11 are also arranged between two pressure plates 17, which are pressed together by the clamping device. Of course, any other suitable clamping device can also be used. To prevent the elastic stack frames 3 from settling due to the clamping pressure, a stop or spacer 18 can also be provided between the end plates 11.Instead of the end plate 11 and the pressure plate 17, of course only a single plate can be used.

[0016] The cell stack 4 is terminated at both axial ends by an end frame 20, which rests against the end plate 11. A metallic current collector 21 is arranged in the end frame 20, for example, in a recess on one end face of the end frame 20, and is connected to an electrical terminal 16. The current collector 21 is made, for example, of copper or aluminum, possibly with an outer coating of an electrically conductive material. The coating is made, for example, of Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, or an alloy thereof. Inorganic compounds, such as oxides, hydroxides, carbides, phosphides, sulfides, borides, etc., or electrically conductive polymers are also conceivable as coatings. Intermediate layers, for example of Ni, between the coating and the base material are also possible. A pantograph 21 could, for example, be made of 200µm aluminum with an intermediate layer of 5µm Ni and an outer coating of 10-100µm Sn.The final frame 20 is described below with reference to the . Fig.3 described in more detail.

[0017] The end frame 20 has a first central recess 22 in which the current collector 21 is arranged. Around the central first recess 22, the end frame 20 forms a radially outer frame 23 with a first end face 24 on the side of the first recess 22, which faces the adjacent stack frame 3. A first sealing element 25 is integrally formed on the first end face 24 of the frame 23 or the end frame 20. This sealing element extends along the circumference of the end frame 20, is closed in the circumferential direction, and rises from the first end face 24. "Integrated" here means that the sealing element 25 is an integral part of the end frame 20 and is formed along with the end frame 20 during the molding process (e.g., injection molding). The sealing element 25 is therefore preferably made of the same material as the end frame 20. In the illustrated embodiment, two such sealing elements 25 are arranged side by side.This first sealing element 25, within the assembled cell stack 4, seals against the electrode plate 5 of the adjacent cell 2 through the applied contact pressure. This contact pressure elastically deforms the sealing element 25, forming an effective seal between the electrode plate 5 and the end frame 20. This seal prevents internal leakage and thus stops electrolyte fluid from coming into contact with the current collector 21. Furthermore, the first end face 24 can seal against the opposite end face 26 of the adjacent stack frame 3 to create an external seal that also prevents electrolyte fluid from escaping from the cell stack 4.

[0018] Another advantageous embodiment of the seal between end frame 20 and adjacent stack frame 3 is described with reference to the Figs. 4 and 5As described, a raised section 27 with a second end face 28 is provided on the side of the first recess 22 at the outer edge of the end frame 20 or frame 23. This section extends around the circumference of the end frame 20. The end frame 20 thus has a central first recess 22 for receiving the current collector 21 and, radially outside the first recess, a second recess 29 with an end face 24 for receiving the electrode plate 5 of the adjacent cell 2. A second sealing element 30 is integrally formed on the second end face 28 of the end frame 20. This sealing element extends along the circumference of the end frame 20, is closed in the circumferential direction, and rises from the second end face 28. In the illustrated embodiment, two such sealing elements 30 are arranged side by side.This second sealing element 30, within the assembled cell stack 4, seals against the opposite end face 26 of the adjacent stack frame 3 through the applied contact pressure. The contact pressure elastically deforms the second sealing element 30, thus forming an effective seal between the end frame 20 and the adjacent stack frame 3, preventing external leakage and therefore the escape of electrolyte fluid from the cell stack 4.

[0019] Likewise, such sealing elements 31 can be provided around the bores 8, 9 on the first end face 24 or second end face 28 to improve the sealing here, as in Fig. 5 depicted.

[0020] In the Fig.6a bis 6e Possible cross-sectional shapes of the sealing elements 25, 30, 31 are shown. The sealing elements 25, 30, 31 can, for example, be triangular ( Fig.6a ), dome-shaped ( Fig.6b ), semicircular or in the form of a circular segment ( Fig.6c ), in the form of a flattened curve or a polygonal path ( Fig.6d ), rectangular ( Fig.6e A sealing element is preferably designed with a width w in the range of 0.1 to 10 mm and with a height h in the range of 5 to 500 µm. If several sealing elements 25, 30, 31 are arranged next to each other, the distance s between them is preferably in the range of 0.5 to 30 mm.

Claims

1. Elastomeric end frame of a redox flow battery with a central first recess (22) to receive a current collector (21) and a radially outer frame (23) with a first end face (24) which surrounds the central recess (22), characterized in that on the side of the recess (22) a first sealing element (25), which extends along the periphery of the end frame (20), is closed in the peripheral direction and protrudes from the first end face (24) of the frame (23), is integrally molded on the first end face (24).

2. Elastomeric end frame according to claim 1, characterized in that a peripheral raised portion (27) with a second end face (28) is provided on the end frame (20) on the radially outer edge, and a second sealing element (30), which extends along the periphery of the end frame (20), is closed in the peripheral direction and protrudes from the second end face (28), is integrally molded on the second end face (28).

3. Elastomeric end frame according to claim 1 or 2, characterized in that a plurality of sealing elements (25, 30) are formed on the first end face (24) or on the second end face (28).

4. Cell stack of a redox flow battery with a plurality of abutting cells (2) formed in each case of two adjacent stack frames (3), wherein adjacent cells (2) are separated from one another by electrode plates (5), and with an end frame (20) according to one of claims 1 to 3, characterized in that the end frame (20) is arranged abutting the adjacent cell (2) at the axial end of the cell stack (4), wherein the current collector (21) in the end frame (20) and the first sealing element (25) abuts the electrode plate (5) of the adjacent cell (2) and the first end face (24) of the end plate (20) abuts the facing end face (26) of the adjacent stack frame (3).

5. Cell stack according to Claim 4, characterized in that the second sealing element (30) abuts the facing end face (26) of the adjacent stack frame (3).