GB2570892A

GB2570892A - Electrochemical cell stack

Info

Publication number: GB2570892A
Application number: GB1802003.2A
Authority: GB
Inventors: Harding Whitehead Adam; Lindsay Underwood Richard; John Ridley Peter
Original assignee: Redt Ltd (dublin Ireland); Redt Ltd Dublin Ireland
Current assignee: Redt Ltd (dublin Ireland); Redt Ltd Dublin Ireland
Priority date: 2018-02-07
Filing date: 2018-02-07
Publication date: 2019-08-14
Anticipated expiration: 2038-02-07
Also published as: WO2019155313A1; GB201802003D0; GB2570892B

Abstract

A flow battery cell stack comprises a plurality of cells, each cell comprising a positive chamber for positive electrolyte, a negative chamber for negative electrolyte, a semi-permeable membrane M separating the positive and negative chambers, a respective bipolar plate BP closing each chamber remote from the membrane and at least one soft frame 101, 102 and at least one hard frame 103 regularly arrayed through the stack, wherein each membrane has a soft frame on at least one side and each bipolar plate has a soft frame on at least one side. The hard frames may be formed from an inert polymeric material, for example high density polyethylene, polypropylene or polyvinylchloride, and the soft frames may be formed from an inert elastomeric material, for example thermoplastic polyolefins. The hard frames may have nesting or inter-engaging features or both, preferably stepped rims or stepped projections, for dimensional stability of the cells. A redox flow battery comprises the flow battery cell stack of the invention.

Description

The present invention relates to a stack of electrochemical or electrolytic cells, in particular though not exclusively to a regenerative reduction/oxidation (redox) fuel cell stack.

Arrangements of electrochemical cells are known which consist typically of between two and fifty alternate positive and negative half cells, although greater numbers are not unknown; since the cells components are stacked together, such a plurality of half cells is typically known as an electrochemical stack or an electrolytic cell stack, often shortened simply to “a stack”. Significant factors in the design of such a cell stack are the method of construction and thickness of the individual cells. Typical arrangements use what is known as a filter press design comprising within each cell successive layers of a non-conductive gasket material. The layers comprise frames, which provide accommodation for electrode material and also contain within their thickness electrolyte flow distribution passages. Each frame is assembled into one of two types of one half cell - positive and negative; it is noted that in general the design of frames for both positive and negative half cells is essentially similar and their assignment as either is a consequence of the overall construction and use of the stack rather than any inherent characteristic. These frames are typically interleaved alternately with sheets of a suitable electrode material and a suitable membrane separator. This construction produces a succession of half-cell pairs in series with electrodes common to two half cells, whence the electrodes are referred to as bipolar electrodes. It is also possible and desirable in some applications to connect electrically to the intermediate electrodes and, depending on the internal electrolyte distribution arrangement, operate the cells in various other series and/or parallel manners when some or all of the electrodes may be unipolar rather than bipolar.

Since the frames must provide a number of different features, including hydraulic sealing, mechanical strength, accommodation of the electrode and flow distribution passages, these passages being required to provide both isolation against internal shunt currents and conversely minimal flow resistance and uniform flow distribution, a design compromise between features is usually required. In particular, it is known to be desirable to achieve high linear flow velocity of electrolyte within the cell, which implies small cell spacing but since the frame thickness defines the spacing this in turn has the undesired effect of reducing the depth available for the distribution passages which are typically indented into one or other surface of the frames. Furthermore, it is known that for efficient and reliable cell performance, closure of the distribution channels within such frames must be achieved such as to prevent undesirable and potentially damaging paths for both hydraulic and electrical current leakage

With a view to providing an improved electrochemical cell stack, in our European Patent Application No 06 726 659, as now granted, (Our Earlier Patent) we have described and claimed an electrochemical stack cell comprising a plurality of cells arranged side-by-side in a stack, each cell having:

• a membrane, • a first half cell cavity on one side of the membrane and a second half cell cavity on the other side of the membrane, • a respective electrode plate at the side of each half cell opposite from the membrane, each electrode plate providing contact between adjacent cells at least for intermediate ones of the cells, • a pair of frames, one for one half cell and the other for the other, the frames:

• captivating the membrane between themselves, • locating the electrode plates and • having:

• continuous margins around central voids providing the half cell cavities, • apertures in the continuous margins providing ducts for flow of electrolyte through the stack for distribution to the cells, • electrolyte distribution rebates at opposite inside edges of the margins and • passages in the continuous margins for electrolyte flow from one of the duct apertures, into and out of the half cell at the distribution rebates and to another of the duct apertures, wherein:

• each plate electrode is captivated between a frame from one cell and a frame from an adjacent cell with at least two portions of the margins of these frames extending outside respective edges of the plate electrode, the adjacent cell frames having faces which abut at the portions;

• the flow passages are formed in the faces of the margins and are closed by abutting opposite frame faces; and • through-frame openings are provided in the frames for extending the passages from the abutting faces of the frames to the other, membrane side of the frames into distribution rebates.

In Our Earlier Patent, we preferred the frames to be rectangular, i.e. having four straight margins, with the electrolyte duct apertures arranged at the comers and the flow passages provided in two opposite margins only.

Also, we preferred to provide all the passages in the face of one of each pair of abutting face frames, i.e. with two passages in each marginal portion having passages with one through frame opening in the portion at the end of one of the passages and another said opening in the other frame opposite the end of the other passage.

Also we preferred for the electrodes to be captivated at rebates in the abutting faces of the frames extending around the entire continuity of the margins around the central void

Further we preferred to provide seals around the ducts and the passages radiating from them and around the electrodes. The seals can be of gasket material, but are preferably O-rings set in grooves in frames.

Two final preferences were:

• passage extensions provided in the opposite faces of the frames from the abutting faces, the extensions extending from the through-frame openings to the respective electrolyte distribution rebates; and • the electrolyte distribution rebates being wider than the electrode captivation rebates.

In our WO 2011/114094, we sought to improve on an inherent shunt current issue with electrochemical cell stacks, by providing that:

The sealing arrangement of both our earlier applications involve multiple Orings in the frame of the cell stack and part of the specific description of our WO 2011/114094 is set out below. This O-ring structure involves the use of many Orings. Their assembly is complex and their cost is significant.

The object of the present invention is to provide an improved electrochemical cell stack.

The Invention

According to the invention there is provided a flow battery cell stack having a plurality of cells, each cell having:

• a positive chamber for positive electrolyte, • a negative chamber for negative electrolyte, • a semi-permeable membrane separating the positive and negative chambers, • a bipolar plate closing the chambers remote from the membrane, the plate on the positive side being shared with the negative chamber of the next cell through the stack and the plate on the negative side being shared with the positive chamber of the next plate of the previous cell through the stack and • at least one soft frame and at least one hard frame;

the soft and hard frames being regularly arrayed through the stack with:

• each membrane having, for sealing of the membrane in the stack, at a soft frame on one side and another of the soft and hard frames on the other side and • each bipolar plate, or a respective hard frame sealingly carrying it, having, for sealing of the bipolar plate in the stack, a soft frame on one side and another of the soft and hard frames on the other side.

Normally the frames will surround the positive and the negative chambers. Preferably the soft and hard parts will be inwardly coterminous around the cell chambers. However, the soft parts could extend inwards of the hard parts to seal to opposed sides of the membranes or the plates and/or possibly the hard frames carrying the plates.

Whilst arrangements can be envisaged where there are two hard parts and two soft parts per cell, normally there will be one hard part and two soft parts per cell. It can also be envisaged that one hard part and one soft part per cell could be feasible.

In the preferred embodiments, the bipolar plates are arranged between soft frames, backed up by hard frames. Nevertheless, they could be arranged between hard frames and sealed to these parts. The sealing could be by the soft frames or otherwise, as by welding or adhesive bonding of the bipolar plates to the hard frames.

The hard parts preferably have nesting and/or inter-engaging features for dimensional stability for cells and the stack. These can be stepped rims. Alternatively, the features can be stepped projections. These can be open and accommodate rods, via which the stack is held compressed for sealing of the soft parts with abutment at the steps controlling separation of the hard parts and the size of the stack.

Conveniently the hard frames are of inert polymeric material and the soft frames are of inert elastomeric material. The relative degree of hardness and softness are such as to enable abutments formed in one part to form seals with abutted areas of the other part. The abutments could be plain, but are preferably ridged, with the soft frames being ridged in preference to the hard frames.

To further assure sealing at the ridges, they can be moulded proud of surrounding plain area of the frames. The ridges can be flat topped but are preferably rounded. They may be flat sided and can be tapered or curved sided.

To help understanding of the invention, specific embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a reproduction of Figure 1 of our WO 2011/114094;

Figure 2 is a reproduction of Figure 6 of our WO 2011/114094;

Figure 3 is a scrap cross-sectional side view similar to Figure 1 of a few cells of a first embodiment of a cell stack in accordance with the invention;

Figure 4 is an enlarged sectional view of ridges of the soft frames of the cell stack of Figure 3;

Figure 5 is a view similar to Figure 3 of a variant of the embodiment of Figure 3;

Figure 6 is similar view of a second embodiment of the invention;

Figure 7 is a similar view of a first variant of the second embodiment;

Figure 8 is a similar view of a second variant of the second embodiment;

Figure 9 is similar view of a third embodiment of the invention;

Figure 10 is a similar view of a first variant of the third embodiment; Figure 11 is similar view of a fourth embodiment of the invention.

Prior Art

Referring to the drawings, Figures 1 and 2, is a reproduction of Figures 1 and of WO 2011/114094 and shows a typical flow battery cell stack arrangement, which we use and refer to as “Our Existing Arrangement”. The corresponding description is 30 as follows:

An electrochemical stack 1 consists of frames of two types, referred to here as passage frames 2 and transfer frames 3. They are arranged alternately in the stack, that is first a passage frame and then a transfer frame along the length of the stack.

On one side or face 4 of each passage frame, it has many grooves for O-ring seals and other grooves for electrolyte passage. These will be described in more detail below. It has a central opening 5 with a rebate 6 open on the one side around the opening. The rebate is half the thickness of an electrode 7 of carbon filled polymer. The rebate has a groove 8 for an O-ring 9 against which the electrode seats. Thus no electrolyte can bypass the electrode. The complementary face 10 of the next transfer frame 3 is plain and has another rebate 11, also half the depth of the electrode. Whilst the electrode is a clearance fit sideways in the rebates, it is captivated by the frames when they are held together, face 4 to face 10. The rebate 11 has no O-ring groove, but face 4 of the passage frame has groove 12 surrounding the rebate 6for an O-ring 14, whereby electrolyte from the rebate 11 cannot flow out sideways between the frames nor by-pass the electrode due to the O-ring 9. The frame 3 also has a central opening 15.

To locate the frames with respect to each other, each passage frame has counter bores 16 in margins around the central opening. The bores extend on through open bosses 17 extending from the plain, other face 18 of the frame with large diameter ends of the counter bores opening in the face 4. Equally the plain, rebatedface 10 of the transfer frame also has open bosses 19, whilst its other face 20 has counter bores 21. With the bosses 19 engaging in the counter bores 16, the frames are fully located against sliding oftheir faces one with respect to the other. With the stack fully assembled, tie bars 22 extend through the bores 16, 21 and exert force to keep the stack tightly compressed together, the tie bars having nuts 35 reacting against stack end plates 36.

It should be understood that the above described frame 2, electrode 7 and frame 3 define within the frames two half-cell spaces 23,24 ofadjacent cells, separated by a common electrode or dipole 7. The spaces are filled with graphite felt 25,26, in electrical contact with the electrode and the electrolyte in the spaces.

Pairs of frames with captivated electrodes are assembled with semi-permeable membranes 30 between them. The top face 20 of each transfer frame has a groove 31 for a membrane sealing O-ring 32. The membrane covers the extent of the O-ring, which presses the membrane against the plain face 18 of the passage frame assembled against it. The membrane is limited in its position lengthways of the frames by fitting between pips 33 on the face 20 of the transfer frame, the pips engaging in bores 33 in the opposite face 15. Laterally, the membrane is limited by the bosses 17 of the passage frame. With the stack compressed, this arrangement seals the membrane to the frames without the possibility of electrolyte escaping from between the frames. A cell is thus defined including the two half-cells on either side of the membrane and the electrodes on opposite sides of the half-cell spaces 23,24.

First Embodiment of the Invention

Turning now to Figures 3 to 11, Figure 3 is a similar view similar to Figure 2 of a stack improved in accordance with the invention.

In common with Our Existing Arrangement, the improved stack has bipolar plates BP dividing individual cells C and semi-permeable membranes M with each cell. The spaces S+ve, S-ve between the membranes and the plates are filled with graphite felt F and respective electrolytes.

In distinction from Our Existing Arrangement, where the frames are hard, albeit with O-rings in grooves, the plates BP are sandwiched, at their edges, between two soft frames 101,102, except that 102 is thicker as will be explained. Otherwise they are essentially mirror images of each other. However they may have differences in passages P for passing electrolyte to and from the respective spaces S+ve, S-ve. Please note that the passages are not described in detail their routing to the respective spaces. They are merely shown by example. In practice they are likely to be similar in layout to the passages shown in Our Existing Arrangement.

The soft frames, in their pairs 101,102, are sandwiched between hard frames 103. All the hard frames are identical. The membranes M are sandwiched between the respective soft frames 102 and the hard frames 103. It is because parts 101 &103 edge the S+ve cell space that the part 102 is thicker than the part 101, because 102 only edges the S-ve cell space. In this way, both cell spaces S+ve, S-ve are of the same thickness.

It is convenient to think in terms of each cell having:

1. One soft frame 101 bounding a positive electrolyte space S+ve, filled with felt

F,

2. One soft frame 102 bounding a positive electrolyte space S-ve, filled with felt F and

3. One hard frame 103 of similar extent to the soft frames and also bounding the positive electrolyte space S+ve,

4. One membrane M captive between the soft frame 102 and the hard frame 103; and the cell sharing:

5. One half of one bipolar plate BP with its neighbour on one side and

6. One half of another bipolar plate BP with its neighbour on the other side.

The hard frames are formed of flats 104 with protrusions 105 to one side. In this embodiment, the protrusions are in the form of hollow mini-turrets 106 with pips 107 at closed ends spaced from the flats. The pips 107 are sized to fit within the opposite open ends 108 of the turrets of the next frames in the stack. Conveniently the turrets are tapered. Typically four turrets are provided in each hard frame, one at each comer. It should be noted that Figure 3 is not a true side elevational crosssection, in that such a cross-section would not pass through the electrolyte spaces. However where turrets intermediate the comers are provided, the cross-section is true.

In addition to the pips, the turrets have end ledges 109, which abut the opposite ends 110 of the next turrets, on the opposite faces of the hard frames. This abutment determines the separation of the individual hard frames through the stack under the clamping action of through rods R.

The soft frames have rebates 112 for accommodating edges 114 of the bipolar plates BP. To avoid leakage from the space S+ve to the space S-ve or vice versa via the rebates, the soft frames are moulded with ridges 115, which are locally compressed when the rods R are tensioned to compress the stack of cells. Compression of the ridges, particularly in view of there being multiple ridges, assures sealing of the soft frames to the bipolar plates. On tensioning of the rods, the compression continues until the turrets abut each other through the extent of the stack.

This states of compression equates to firm contact between the soft and hard frames where, away from the ridges, they are formed with flats 11 <5,117. Equally the opposed soft frames have flats 118, which abut. Such abut provides a degree of sealing, which is augmented by the ridges. To ensure that the ridges 115 are sufficiently compressed for them to form seals they are higher than they would be if they merely touched the plates BP. Typically the degree of extra height is of the order of 0.5mm. Further space 119 is provided to either side of each ridge, to allow displacement of the compressed material. According to the depth of the space 119, the bottom of it may remain clear of the bipolar plate BP, or may be pressed against it to further assure sealing.

Where each membranes M is sandwiched between a respective soft frame 102 and a respective hard frame 103, the soft frame has ridges 120. The hard frame has a shallow rebate 121 in which the membrane is located. These ridges 120act not only to seal the membrane to the soft frame but compression between them but also to seal the membrane to the hard frame, the membrane being compressed against this part opposite the ridges, but to the flexibility of the membrane. Thus electrolyte leakage around the edges of the membrane is avoided.

As shown in Figure 3, the soft frames 101,102 have passages P for electrolyte circulation. These passages are formed at the interface between the soft frames and the hard frames. To ensure electrolyte sealing from escaping these passages, soft frames are formed with further ridges 122 arranged to abut the hard frames 103 at the passages against electrolyte leakage out of the passages. Yet further ridges 123 are provided around the inner margin of the cell spaces again to guard against electrolyte leakage between the hard and soft frames.

As shown in Figure 4, the ridges 115,120 are moulded with a pointed crosssectional shape 125, which is pressed down to a more rounded shape 126 when the cell stack is full closed up. This enhances the sealing efficacy of the ridges.

Referring now to Figure 5, a variant of the first embodiment is shown, in which the hard frame 1031 does not extend in so far as the two soft frames 1011,1021. These have opposed membrane sealing ridges 1021. The membrane M is located laterally against the edge of the respective hard frame 1031 and sealed on both faces by the ridges 1211. Thus leakage between the two electrolyte spaces is avoided. The bipolar plates BP are sealed to the soft frames by ridges 1151. It will be noted that the arrangement is symmetrical, whereby the soft frames 1011,1021 can be mirror images of each other.

In this variant, the turrets 1061 and their pips 1071 are fully hollow and of larger diameter allowing the rods R to pass through them.

Second Embodiment of the Invention

Referring on to Figure 6, a second embodiment of the invention is shown in which soft frames 201,202, via ridges 220 captivate the membrane and seal it.

On their other sides, the soft frames 201,202 have further ridges 215 for captivating and sealing the bipolar plates. The latter are rebated 233 at their edges, which are between the soft frames. The hard frames 203 are similarly rebated 234. Thus the plates are located during build-up of the stack. The plates and hard parts can be preliminarily bonded together. To further assure sealing, the hard frames can be provided with ribs 235 for abutting the bipolar plates with interference force.

Alternatively, as shown in Figure 7, the hard frames and the bipolar plates can be welded together, as by traversing a laser around their plain joint 236 to locally melt their polymeric materials together at a weld 237.

In this alternative and the second embodiment, the soft frames 201,202 extend further in than the hard frames 203. In the second variant as shown in Figure 8, the hard frames extend further in. They and the plates are rebated and welded, for intercell sealing, between the cell spaces S+ve, S-ve. Another possibility is that the rebates are at the region of the ridge seals 220, which seal on both the plates and the hard frames, as indeed shown in the next embodiment.

Third Embodiment of the Invention

Whilst the first and second embodiments use two soft frames and one hard frame per cell, the third embodiment, shown in Figure 9, use two soft frames and two hard frames per cell.

The cell’s membrane is housed between one soft frame 301 with ridges 320 and one hard frame 3031. The hard frame is otherwise plain and has the other soft frame 302 on its other side, sealing to it by ridges 323. The other hard frame 3032 is the same thickness as the bipolar plate. They can be rebated or welded as in the previous embodiment. Soft frames have ridges 315 for sealing against both sides of the hard frame and the bipolar plate.

In a non-illustrated variant, the hard frame 3032 is thinner than the bipolar plate and the soft frames are rebated to both abut the hard frame and the bipolar plate - at the ridges.

In a different configuration of Figure 10, two hard frames 3033,3034 these are mirror images, at the bipolar plate, having rebates 312 for the plate. The soft frames 3013,3023 are have lips 341, which over hang the hard frames, to seal on the bipolar plates, with ridges 3153. On their other sides, they have ridges 3203 for sealing the membrane M.

Fourth Embodiment of the Invention

By contrast the fourth embodiment of Figure 11 has one soft frame 401 and one hard frame 403 per cell. The latter is rebated 412 to accommodate the bipolar plate equally spaced from the membranes on their opposite sides. The soft frame has ridges 420,423 for sealing against the membrane M and the hard frame.

It will be appreciated that all the above embodiments and variants have ridges for sealing against the membranes and the bipolar plates. This enables all of the seals to be produced by injection moulding of the soft frames, without the need for assembly of separate seal elements, such as O-rings, to the frames. The ridges can be of differing heights and to suit the stiffness of the soft frames. Typically, the ridges will be higher where the material is softer and vice versa. Softer material enables greater tolerances in the dimensions of the parts to be accommodated, whereas more predictable compression without sideways deviation is to be expected with harder materials.

It will be appreciated that communication with the passages P from ducts through the stack and to the electrode spaces is not shown in the above embodiments, but can be effected in know ways, i.e. by frame grooves and through bores. It is envisaged that these will usually be in the soft frames, but could be in the hard frames. Indeed aligned grooves in both the hard and the soft frames may be provided in the interest of enlarging flow cross-section.

Typically hardnesses in the range 25 Shore A to 90 Shore A and preferably 40 Shore A to 80 Shore A are anticipated to be acceptable for the soft frames, whereas 60 Shore D to 100 Shore D are likely to be useable for the hard frame members. It should be noted that the scales overlap in that 90 Shore A is roughly equal to 40 Shore D.

Typical soft materials to be considered for frames include thermoplastic polyolefins (TPO) - a blend of polyolefinic thermoplastic such as PP and/or PE with an elastomeric component such as EPR and/ or EPDM. Also some thermoplastic vulcanizates (TPV) can be used, especially polyolefin-based varieties. Santoprene™ is a typical TPV brand. Other soft materials that may be used include polyisoprene (IR), ethylene propylene diene rubber (EPDM), chlorosulfonated polyethylene (CSM, e.g. Hypalon™) and fluoroelastomers (FPM/ FKM, e.g. Viton™).

Typical hard materials include polyethylene (PE) - especially high density polyethylene (HDPE), polypropylene (PP), polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC), fluoropolymers (e.g. ETFE, PVDF, PTFE, etc.) and polystyrene (PS).

The invention is not intended to be restricted to the details of the above described embodiment. For instance, the membranes could be adhesively attached to either of the soft or hard frames prior to assembly of the cell stack. It is possible that such adhering could be sufficient for sealing of the membranes at least to the frame to which it is initially adhered. Further in our WO 2011/114094, the frame parts are referred to as passage frames and transfer frames. In the embodiments of the invention, the passages P are in soft frames, equating them with the passage frames. However, it is possible for the passages P to be in the hard frames.

Claims

1. A flow battery cell stack having a plurality of cells, each cell having:

• a positive chamber for positive electrolyte, • a negative chamber for negative electrolyte,

5 · a semi-permeable membrane separating the positive and negative chambers, • a respective bipolar plate closing each chamber remote from the membrane, the plate on the positive side being shared with the negative chamber of the next cell through the stack and the plate on the negative side being shared with the positive chamber of the previous cell through the stack and

10 · at least one soft frame and at least one hard frame;

the soft and hard frames being regularly arrayed through the stack with:

• each membrane having, for sealing of the membrane in the stack, a soft frame on one side and another of the soft and hard frames on the other side and • each bipolar plate, or a respective hard frame sealingly carrying it, having, for

15 sealing of the bipolar plate in the stack, a soft frame on one side and another of the soft and hard frames on the other side.

2. A flow battery cell stack as claimed in claim 1, wherein the frames surround the positive and the negative chambers.

3. A flow battery cell stack as claimed in claim 1 or claim 2, wherein the soft and

20 hard frames are inwardly coterminous around the cell chambers.

4. A flow battery cell stack as claimed in claim 1 or claim 2, wherein the soft frames extend inwards of the hard frames to seal to opposed sides of the membranes or the plates and/or possibly the hard frames carrying the plates.

5. A flow battery cell stack as claimed in claim 1, claim 2, claim 3 or claim 4,

25 wherein there are two hard frames and two soft frames per cell.

6. A flow battery cell stack as claimed in claim 1, claim 2, claim 3 or claim 4, wherein there are one hard frame and two soft frames per cell.

7. A flow battery cell stack as claimed in claim 1, claim 2, claim 3 or claim 4, wherein there are one hard frame and one soft frame per cell.

30

8. A flow battery cell stack as claimed in preceding claim, wherein the bipolar plates are arranged between soft frames, backed up by hard frames.

9. A flow battery cell stack as claimed in any one of claims 1 to 7, wherein the bipolar plates are arranged between hard frames and sealed to these frames, preferably

1708 18 by the soft frames by welding or adhesive bonding of the bipolar plates to the hard frames.

10. A flow battery cell stack as claimed in preceding claim, wherein the hard frames have nesting and/or inter-engaging features for dimensional stability for cells and the

5 stack, the features preferably being be stepped rims or stepped projections.

11. A flow battery cell stack as claimed in claim 10, wherein these projections are open and accommodate rods, via which the stack is held compressed for sealing of the soft frames, the projections preferably having abutments at the steps controlling separation of the hard frames and the size of the stack.

10 12. A flow battery cell stack as claimed in preceding claim, wherein the hard frames are of inert polymeric material and the soft frames are of inert elastomeric material. 13. A flow battery cell stack as claimed in preceding claim, wherein the relative degree of hardness and softness are such as to enable abutments formed in one frame to form seals with abutted areas of the other frame.

15 14. A flow battery cell stack as claimed in claim 13, wherein the abutments are plain.

15. A flow battery cell stack as claimed in claim 13, wherein the abutments are ridged, with the soft frames being ridged in preference to the hard frames.

16. A flow battery cell stack as claimed in claim 15, wherein the ridges are proud of surrounding plain area of the frames, for assurance of sealing at the ridges.

20

17. A flow battery cell stack as claimed in claim 15 or claim 16, wherein the ridges are flat topped or rounded and flat sided or tapered or curved sided.

18. A flow battery cell stack as claimed in preceding claim, wherein each membrane has a soft frame on one side and another soft frame on the other side.

19. A flow battery cell stack as claimed in any one of claims 1 to 17, wherein each

25 membrane has a soft frame on one side and a hard frame on the other side.

20. A redox flow battery comprising a flow battery stack as claimed in any preceding claim.

Intellectual Property Office