Method of protecting a weld on an electrochemical cell

Abstract

The invention relates to welding an electrochemical cell assembly. The method comprises providing a first and a second metal sheet to be welded together; applying a controlled corrosion treatment to an area to be situated between the first and second metal sheets; and welding the first and second metal sheets together at a location offset from the area in which the controlled corrosion treatment was applied.

Description

[0001] Title: Method of protecting a weld on an electrochemical cell

[0002] Specification

[0003] The invention relates to the field of electrochemical cells which are adapted to be arranged in stacks, in particular, fuel cell stacks and electrolyser cell stacks.

[0004] Fuel cell units and electrolyser cell units are examples of electrochemical cell units. Fuel cell units are energy conversion devices that allow for conversion of electrochemical fuel to electricity. Electrolyser cell units may be considered fuel cell units running in reverse mode, i.e. using electricity to decompose a compound into its constituent parts, for example water into hydrogen and oxygen. Reversible cell units are capable of operating in both modes.

[0005] Metal supported electrochemical cell units comprise a metal cell layer having electrochemically active layers (a substrate) and an interconnector plate. The substrate and the interconnector plate may be sealingly attached to one another and define a fluid volume therebetween. The seal enclosing the fluid volume may be provided by welding the cell layer and interconnector plate together.

[0006] Metal-supported solid oxide fuel cell (SOFC) or solid oxide electrolysis cells (SOEC) typically operate in a temperature range between 500 to 1000 °C. This temperature range together with the process gases used (steam, hydrogen, methane, air) are very critical in terms of high temperature corrosion (oxidation) in particular of components which are made of stainless steel, e.g. the interconnect and the substrate, but also other components. For that reason, it is common to protect the surface of the metal components by a protective layer. This can be achieved by an external coating (e.g. CVD, PVD), a pre-oxidation of an uncoated steel (leading to a naturally grown protective oxide layer) in air at temperatures > 800 °C, or a combination of both (metal coating followed by a pre-oxidation to form a protective oxide (multi)layer(s)).

[0007] However, the corrosion protective layers / coatings are locally destroyed during the welding leading to an accelerated corrosion of welded joints in contact with air. The inventors have found that this is a particular problem at regions where the weld seam is relatively thin (e.g. the connection between two sheets) and has been found when an interconnect is welded to a substrate (see Fig. 2). The corrosion of this weld can lead to a gas leakage (cf. air side and fuel side) and thus to premature loss of performance or failure of the complete stack. It is an object of the present invention to ameliorate at least some of these problems thus extending the life and / or performance of an electrochemical cell.

[0008] According to the invention, there is provided a method of welding an electrochemical cell assembly according to claim 1. The method comprises optionally providing a first and a second metal sheet to be welded together. These sheets may correspond to a substrate and an interconnect respectively. The method further comprises applying a controlled corrosion treatment to an area to be situated between the first and second metal sheets to be welded together. The first and second metal sheets are then welded together at a location offset from the area in which the controlled corrosion treatment was applied.

[0009] In such a way, a corrosion barrier is introduced which acts to inhibit fluid penetration through the gap between the metal sheets to the weld. This protects the weld from the fluid (which may be air, or another oxidising agent), thus reducing the likelihood (and severity) of the weld becoming corroded and introducing leak pathways.

[0010] Optionally, the controlled corrosion treatment comprises decoating a protective layer of one or both of the first and second metal sheets. This induces corrosion at this area, forming a protective corrosion layer offset from the weld.

[0011] In one example, the decoating comprises scratching. This is a simple method which can be implemented in an automated production line.

[0012] In another example, the decoating comprises laser treatment to remove the protective layer. Such a process can be finely tuned and is suited to high speed automation. It should be appreciated that both scratching and laser treatments may be applied, and that the different sheets may have different decoating methods applied.

[0013] Optionally, the controlled corrosion treatment comprises applying a material containing a corrosive material to the area to be situated between the first and second sheets. In a similar manner to decoating, the corrosive material induces corrosion at this area, forming a protective corrosion layer offset from the weld.

[0014] Optionally, the controlled corrosion treatment comprises applying a material containing a material susceptible to corrosion to the area to be situated between the first and second sheets. This method differs from the above insofar as the protective coating on one or both metal sheets are not directly modified, rather an additional material is introduced to provide a protective corrosion layer offset from the weld. The corrosive material, or material susceptible to corrosion may comprise a paste. This can be accurately dispensed and is suited to an automated production line. Alternatively, or in addition, the material susceptible to corrosion may comprise a foil or wire.

[0015] The material susceptible to corrosion may comprise iron or steel. Such materials are susceptible to oxidation.

[0016] In a preferred example, the weld is provided at a periphery of the metal sheets. Preferably wherein the weld encloses a fluid volume between the first and second metal sheets.

[0017] Optionally, one or both metal sheets are tub shaped. This is a convenient manner in which to enclose a fluid volume yet provide an area (a flanged perimeter) in which the metal sheets are parallel for welding.

[0018] Optionally, the area in which the controlled corrosion treatment is applied circumscribes the enclosed fluid volume. In such a way, a controlled corrosion barrier is formed around the enclosed fluid volume.

[0019] Optionally, the weld is offset from the area in which the controlled corrosion treatment was applied in a direction towards the enclosed fluid volume. In such a way, the corrosion barrier is situated so as to inhibit fluid from outside the enclosed fluid volume from contacting the weld. In many use cases, the fluid outside the enclosed fluid volume is air (or another oxidant).

[0020] In another aspect of the invention there is provided an electrochemical cell unit manufactured according to the method of claim 1.

[0021] In a further aspect of the invention, there is provided a stack of electrochemical cell units so arranged to form a fuel cell stack or electrolyser stack.

[0022] Further embodiments are derivable from the following description and the drawings:

[0023] Figure 1 shows an example electrochemical cell with an example weld path;

[0024] Figure 2 shows a cross section of a weld exhibiting corrosion damage;

[0025] Figure 3 shows a flow diagram of a method of welding an electrochemical cell assembly; and

[0026] Figures 4a and 4b show cross-sections of an electrochemical cell with a protective treatment applied.

[0027] Figure 1 shows an exploded view of an example electrochemical cell unit 10, and two gaskets 34. Further details relating to the design and operation of electrochemical cells and stacking them to form a fuel cell (or electrolyser) can be found in WO2020 / 126486. As can be seen, the electrochemical cell unit 10 comprises a flat (i.e. planar) metal support plate (substrate) 14 stacked next to a separator plate 12 - in this case above it. The separator plate 12 is shown to have flanged perimeter features 18 around its perimeter.

[0028] The flanged perimeter features 18 extend out of the predominant plane of the sheet, as found at a central fluid volume area, to create a concavity in the separator plate (and a convexity to the outside surface). The concavity will form the fluid volume 20 within this fuel cell unit upon assembly of the fuel cell unit.

[0029] In this illustrated arrangement (simplified to illustrate key features of the invention), the fuel cell unit 10 has rounded ends and parallel sides, with a fluid port 22 towards each end. Other shapes, dimensions and arrangement of fluid ports are possible.

[0030] In a middle portion of the fuel cell unit 10, an electrochemically active layer 50 is provided on the metal support plate. In this embodiment it is located outside of the fluid volume 20. Dimples 24 around the fluid ports 22 are provided to provide mechanical stability to the enclosed fluid volume.

[0031] In one embodiment, the electrochemically active layer 50 comprises an anode deposited on the substrate, then an electrolyte and then a cathode. In fuel cell mode, the enclosed fluid volume delivers fuel (hydrogen) to the electrochemically active layer 50 via pores or holes in the substrate. In electrolysis mode, the fuel is steam which passes through the pores or holes in the same manner.

[0032] A weld is provided all away around the perimeter of the electrochemical cell unit 10 on the flanged perimeter to enclose a fluid volume. Laser welding is a preferred welding method for accuracy and speed. However, welding completely destroys the protective layers on the metal sheets, which leaves the weld site susceptible to corrosion. This problem is particularly acute when the weld seam is relatively thin (e.g. the connection between two sheets) as it is found in the electrochemical cells when the interconnect is welded to the substrate.

[0033] Figure 2 shows a cross section of a weld exhibiting corrosion damage. In this image, the thickness of the weld 40 is 84 pm. From Fig. 2 it can be seen that the weld is only slightly corroded since the gap at air side between substrate and interconnect is filled with corrosion products (oxides) preventing air to reach the welding seam (Fig. 2, left side). However, since the gap size can vary and corrosion in the gap is an uncontrolled process, it cannot be guaranteed that the whole gap is closed by corrosion products preventing the weld seam from corrosion.

[0034] To prevent critical corrosion of the weld seam caused by corrosive media e.g. air penetration through the gap of the jointed weld the space within the gap is filled with corrosion products initiated by controlled corrosion between both metal sheets. The corrosion products between the metal sheets prevent the weld seem located in the gap between both sheets from corrosion.

[0035] Figure 3. shows a flow diagram of a method of welding an electrochemical cell with weld corrosion protection.

[0036] The first step SI is to provide first and second metal sheets to be welded together. As discussed above, the problem being addressed is particularly acute when the metal sheets correspond to an interconnect and a substrate, but it could be in other areas, such as the base plate (or top plate) of a stack of electrochemical cells and the 'skirt' which contains the air within the stack.

[0037] Step S2 is to apply a controlled corrosion treatment to an area between the metal sheets. Specifically, an area on one or both metal sheets which will face each other when welded together. This area preferably corresponds to a weld path, and in one example circumscribes the fluid volume enclosed by the two metal plates. In particular, the area in which the corrosion treatment is applied circumscribes the weld path (which in turn circumscribes the enclosed volume). Figures 4a and 4b (described below) provide two different examples of a controlled corrosion treatment.

[0038] At step S3 the metal sheets are welded together. This may be achieved by laser welding, or other suitable process depending on the characteristics of the metal and processing environment. The weld is performed at a position offset from the area in which the controlled corrosion treatment was applied, thereby introducing a corrosion barrier which acts to inhibit air penetration through the gap between the metal sheets to the weld.

[0039] The method should be simple and cheap to perform, particularly in a high-speed, automated production line. Figures 4a and 4b show two example embodiments which show the result of two different methods of applying a controlled corrosion treatment to an area between the metal sheets

[0040] Figure 4a shows local decoating 42 of the protective layer / coating of one or both metal sheets 12, 14 at the same distance away from the weld 40. Decoating can be achieved by scratching or by a laser. By doing so, the area which has been decoated corrodes first and a protective corrosion barrier is produced, inhibiting the air (or other oxidising fluid) from corroding the weld.

[0041] Figure 4b shows the gap being filled with a paste, foil or wire 44 containing / composed of a corrosive medium (e.g. iron or steel). This material may be corrosive so as to induce corrosion on the metal sheets 12, 14 themselves, or be susceptible to corrosion thus forming a protective corrosion barrier.

[0042] In both embodiments shown, air penetration through the gap to the weld 40 and thus corrosion of the welding seam is prevented, or significantly reduced. In embodiments where the weld 40 encloses a fluid volume, it is beneficial to offset the weld from the area in which the corrosion treatment is applied towards the enclosed fluid volume. This means the protective corrosion barrier is situated on the 'air side' of the weld, meaning the weld is protected from corrosion (oxidation). If the oxidising agent is on the opposing side, the relative positioning of the weld and protective corrosion barrier should be reversed.

Claims

Claims1. A method of welding an electrochemical cell assembly, the method comprising: applying a controlled corrosion treatment to an area to be situated between a first and a second metal sheet to be welded together; and welding the first and second metal sheets together at a location offset from the area in which the controlled corrosion treatment was applied.

2. The method according to claim 1 wherein the controlled corrosion treatment comprises decoating a protective layer of one or both of the first and second metal sheets.

3. The method according to claim 2 wherein decoating comprises scratching.

4. The method according to claim 2 or 3 wherein decoating comprises laser treatment.

5. The method according to any preceding claim wherein the controlled corrosion treatment comprises applying a corrosive material to the area to be situated between the first and second sheets.

6. The method according to any preceding claim wherein the controlled corrosion treatment comprises applying a material containing a material susceptible to corrosion to the area to be situated between the first and second sheets.

7. The method according to claim 6 wherein the material susceptible to corrosion comprises a paste.

8. The method according to claim 6 or 7 wherein the material susceptible to corrosion comprises a foil or wire.

9. The method according to any of claims 6 to 8 wherein the material susceptible to corrosion comprises iron or steel.

10. The method according to any preceding claim wherein the weld is provided at a periphery of the metal sheets.

11. The method according to any preceding claim wherein the weld encloses a fluid volume between the first and second metal sheets.

12. The method according to the preceding claim wherein one or both metal sheets are tub shaped.

13. The method according to any of claims 10 to 12 wherein the area in which the controlled corrosion treatment is applied circumscribes the enclosed fluid volume.

14. The method according to any of claims 10 to 13 wherein the weld is offset from the area in which the controlled corrosion treatment was applied in a direction towards the enclosed fluid volume.

15. The method according to any preceding claim wherein the first metal sheet is a substrate and the second metal sheet is an interconnect.

16. An electrochemical cell assembly manufactured using a method according to the preceding claims.

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