Field of the Invention

The present invention relates to electrochemical cell units with a flat separator, in particular, fuel cell units and electrolyser cell units, stacks containing such cell units, methods for manufacturing a separator plate (interconnect) for use in such cell units, separator plates so formed, and the use of such cell units. The cell units 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) units, and these may include metal- supported solid oxide fuel cell (MS-SOFC) or electrolyser cell (MS-SOEC) units.

Background to the Invention

Some electrochemical cell units can produce electricity by using an electrochemical conversion process that oxidises fuel to produce electricity. Some electrochemical cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as electrolyser cell units, for example to produce hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide. They may be tubular or planar in configuration. Planar electrochemical cell units may be arranged overlying one another in a stack arrangement, for example 100-200 electrochemical cell units in a stack, with the individual electrochemical cell units arranged, for example, 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 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 an electrochemical cell unit and concerns the design of separator plates for them. 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.

Each cell unit in a stack of cell units typically includes a cell layer comprising an electrochemically active cell region (such as a metal-supported electrochemically active cell region) and a separator plate. A separator plate typically contacts one side of the cell layer of a cell unit and, in a stack of cell units, and may also contact an opposite side of a cell layer of an adjacent cell unit. A separator plate that, in a stack of cell units, contacts one side of the cell layer of its cell unit and an opposite side of a cell layer of an adjacent cell unit may be referred to as an "interconnect”.

Figure 1 shows an exploded perspective view of a cell unit with two gaskets, taken from the Applicant's earlier application GB 2603665 A, which discusses an electrochemical cell unit and a stack comprising a plurality of such electrochemical cell units with raised elements. The cell unit 10 of Figure 1 comprises a flat (i.e. planar) metal support plate 14 stacked next to a separator plate 12. The separator plate 12 is shown to have flanged perimeter 18 around its perimeter. The flanged perimeter 18 extends 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 a fluid volume within this cell unit upon assembly of the cell unit.

In the illustrated arrangement in Figure 1 the cell unit 10 has rounded ends and parallel sides, with a fluid port 22 towards each end in both the separator plate 12 and the metal support plate 14. Other shapes and sizes and numbers of the respective cell features are possible depending upon the required power and dimensions of the final stack assembly.

Around the fluid ports of the separator plate 12, shaped port features 24 are provided. The shaped port features 24 are provided as multiple elements in the form of round dimples extending out of the plane of the base of the fluid volume a distance corresponding to that of the height of the flanged perimeter 18 - to have a common height therewith. This is so that they will contact the opposing surface of the metal support plate 14, just like the flanged perimeter 18, when the cell unit 10 is assembled. As a result, when the flanged perimeter 18 is joined to the metal support plate 14, for example by welding, the shaped port features 24 will likewise contact the metal support plate 14.

In a middle portion of the cell unit 10, an electrochemically active layer 50 is provided on the metal support plate. In this example it is located outside of the enclosed fluid volume.

The electrochemically active area 50 includes an anode, a cathode, and an electrolyte positioned between the anode and cathode (not shown). The anode, electrolyte, and cathode may together be referred to as the electrochemically active layer 50, active electrochemical cell layer, or electrochemically active region. The electrolyte conducts either negative oxygen ions or positive hydrogen ions between the anode and cathode. The stack 20 may comprise a stack of cell units that are based on one of solid oxide electrolytes, polymer electrolyte membranes, or molten electrolytes or any other variant capable of electrochemistry.

The concave configuration can give the relevant plate the appearance of a rimmed tray, with a correspondingly convex outside shape (outside relative to the cell unit) and usually a planar base, the concavity thus defining (e.g. part of) the fluid volume in the assembled cell unit. In this concave configuration, the flanged perimeter extends out of a plane of the original sheet of the separator plate, and/or of the metal support plate, toward a respective opposed surface of the other of the separator plate and the metal support plate.

The fluid volume is thus bordered by a flanged perimeter, which is formed by pressing, such as by use of a die press, hydroforming or stamping. These are simple processes that are already being undertaken in the formation of central protrusions in the fluid volume (described below), as found likewise on the separator plate in the prior art, for supporting and electrically connecting adjacent cells via the electrochemically active layers.

Figure 2 shows an exploded perspective underside view of the cell unit of Figure 1. The metal support plate 14 (e.g. metal foil) is provided with multiple small holes or pores 48 to enable fluid in the fluid volume to be in contact with the side of the electrochemical layers that is closest to the metal support plate 14. These form a porous region bounded by a non-porous region. The anode (fuel electrode) layer is located adjacent the small holes/pores with the (enclosed) fluid volume within the cell unit comprising a fuel flow volume supplied by fuel entering and exiting via the fluid ports 22, which are thus fuel ports 22. The cathode (air electrode) layer is on the opposite side of electrochemically active layer 50, i.e. on its outer face, and is exposed to air flowing across that layer during use of the cell unit 10.

In the cell units depicted in Figure 1 & 2 only two layers (components) are required, i.e. the metal support plate and the separator plate.

There are also provided central upward protrusions 32 and central downward protrusions 36 which include in and out protrusions (up and down as shown), extending between the internal opposed surfaces of the two plates and an outer surface of the electrochemically active layer of the cell unit adjacent to the outward protrusions. The central upward protrusions 32 define fluid pathways between them or in them for fuel, the pathways being through the enclosed fluid volume between fluid ports at each end of the cell unit. The central downward protrusions 36 define fluid pathways between them, or in them for oxidant (such as air) through the fluid volume defined between the outer surface of the electrochemically active layer of the cell unit adjacent to the downward protrusions.

Each gasket, for example gasket 34, (also referred to as a "seal") provides a primary sealing function and will usually be a compressible gasket that is subjected to high compressive forces in the vicinity of the ports.

The gaskets may be sized to cover all the shaped port features 24 of each fluid port 22 to prevent fluid (such as fuel) that may be travelling through the fluid ports 22 in a stack from seeping between the outside of the cell unit 10 and the gaskets (for example gasket 34), into the area external of the cell units, i.e. into the fluid surrounding the cell units 10 (such as oxidant), or the fluid external of the fluid ports from seeping in the other direction - into the fluid ports. This is important to prevent any mixing of the fluid inside the cell unit 10 and the fluid outside the cell unit 10, which will be fuel and oxidant. The polarity of the electrochemically active layers 50 determines which way round this will be.

The gaskets may also provide electrical insulation between a first cell unit 10 and an adjacent fluid cell unit 10, so as to prevent a short circuit. The gaskets may be any suitable cell gaskets (sealing rings), such as, for example, vermiculite-based gaskets, eg Thermiculite (trade mark).

Cell stacks have various sources of internal resistance. One such source is contact resistance between the separator plate and an adjacent cell layer. A cell stack may have a top compression plate and a bottom compression plate connected together by bolts or other means to allow cell unit(s) therebetween to be compressed together. The compressive force applied to the stack is sufficient to create a seal to prevent seeping out of the cell unit(s) and/or prevent fluid external of the fluid ports from seeping into the fluid ports.

Compressive forces in the stack within a plan view area of the electrochemically active region are required for good electrical contact and hence good conductivity through the stack. The central upward protrusions 32 and central downward protrusions 36 create the required electrical contacts between cell units and also provide a support function for the cell unit in the central region, extending upwardly to the underside of the metal support plate 14 at the area of the small holes or pores 48, and downwardly to the opposing surface of the electrochemically active layer of a cell below it. In addition, shaped port features 24 around the ports 22 assist in the transfer of compressive forces in the stack at the peripheral ends of each unit cell, to provide the compressive force required to create a seal. There is a need to maintain pressure between the separator plate and the adjacent cell layer to minimize contact resistance between the separator plate and an adjacent cell layer. This is a function of the upward protrusions 32. However, the inclusion of such upward protrusions 32 also has its own disadvantages. For example, the upward protrusions 32 can obstruct holes or pores 48 in the metal support plate 14 and obstruct fuel flow to the electrochemically active cell area. In effect, they detract from efficiency and power density of the cell unit by reducing access of fluid to (and exhaust of product from) the electrochemically active cell region via the pores 48. The skilled person will understand that the electrode fed by the pores 48 is itself adapted to transfer reactant to the electrolyte, and so pores blocked by the protrusions 32 reduce supply to (and exhaust from) the electrochemically active cell region, but do not render the portion of the electrochemically active cell region proximal to a blocked pore inoperable. The upward protrusions 32 also can obstruct to the flow of fluid (such as fuel) across the cell unit, and reduces the capacity of the volume for the flow of said fluid.

The present invention seeks to address, overcome or mitigate at least one of the prior art disadvantages.

Summary of the Invention

In a first aspect, there is provided an electrochemical cell unit comprising: a cell layer comprising an electrochemically active cell area, the cell layer having a first side (e.g. lower side) and a second side (e.g. upper side), and a separator plate (e.g. below the cell layer) having a first side (e.g. lower side) and a second side (e.g. upper side), the separator plate comprising a metal sheet, the second side of the separator plate extending across and facing the first side of the cell layer in a spaced arrangement to form a first fluid volume for first fluid therebetween. The separator plate has a region that extends across (e.g. beneath) at least the electrochemically active cell area that is clear or substantially clear of protrusions directed toward the first side of the cell layer (i.e. clear of protrusions protruding into the first fluid volume). This region is substantially free (preferably entirely free) of other components to separate the separator plate from the cell layer. The separator plate is adapted to be exposed to a pressure difference between the first side and the second side of the separator plate to maintain the spaced arrangement that forms the first fluid volume. Preferably, the pressure difference is a fluidic pressure difference, more specifically, gaseous pressure difference.

The second side of the separator plate extends across the first side of the cell layer in an underlying/overlying arrangement. In the figures, the latter overlies the former.

The region that extends across the electrochemically active cell area is entirely or almost entirely flat and largely, almost entirely or entirely free from protrusions or raised features directed toward the first side of the cell layer (of the cell unit of which the separator is a constituent). Such protrusions may include channels, ridges, or dimples and may typically be formed by pressing, etching, or machining. There is no support structure in the first volume to maintain that volume.

The region that extends across the electrochemically active cell area may be coincident with the plan view area (i.e. extent) of the electrochemically active cell area. In other words, the second side of separator does not contact first side of cell layer within the plan view area (i.e. extent) of the electrochemically active cell area and the region is clear of other components to separate the separator plate from the cell layer.

In an operational mode of the electrochemical cell unit, a pressure difference between the first side and the second side of the separator plate (i.e. a positive pressure difference between the first fluid volume and the second fluid volume) maintains or increases a separation between the second side of the separator plate and the first side of the cell layer. In a non-operational mode, when the pressure on each of the first and second sides of the separator plate is the same, the separation may decrease.

Preferably, the architecture of the cell layer is selected from one of the following: metal-supported, anode-supported, electrolyte-supported, or cathode-supported architecture. That is to say the cell layer is one of a metal-supported cell layer, an anode-supported cell layer, an electrolyte-supported cell layer, or a cathode-supported cell layer.

More preferably, the cell layer is a metal-supported cell layer and the first side of the cell layer is a first side of a metal support plate and the second side of the cell layer is a second side of the metal support plate opposite the first side of the metal support plate, the second side carrying the electrochemically active cell area. Furthermore, any mention of a cell layer throughout the description is interchangeable with a cell layer supported by a metal support plate, or a "metal plate supported cell layer" or such like.

The electrochemical cell unit further comprises an inlet to and an outlet from the first fluid volume, preferably positioned towards opposing edges of the cell unit with the electrochemically active cell area positioned therebetween. The inlet to the first fluid volume may be a type of port for the flow of a fluid (such as a reformed fuel) into the first fluid volume formed by the spaced arrangement between the cell layer and the separator plate. The outlet from the first fluid volume also may be a type of port for the flow of a fluid (such as a reformed fuel) into the first fluid volume formed by the spaced arrangement between the cell layer and the separator plate.

The electrochemical cell unit preferably comprises a first plurality of protrusions outwardly extending from the first side of the separator plate, away from the cell layer. The protrusions being raised features or components of the separator plate, either attached or integrally formed with the separator plate. When the protrusions are integrally formed with the separator plate they may be formed through pressing of the separator plate. Preferably, the first plurality of protrusions is in a region that overlies at least the electrochemically active cell area of the cell unit.

The protrusions may have a circular, square, cross, pentagonal, or hexagonal-shaped cross-section. They may also be oval or irregular polygon in cross-section, but ideally should have a lateral-to- vertical aspect ratio less than 10, preferably less than 5, more preferably less than 2. Alternatively or additionally, the length of any protrusion may be less than half of a characteristic lateral dimension (e.g. length, width, or diameter) of the electrochemically active cell region.

One or both of the separator plate and the cell layer of the electrochemical cell unit may also be provided with a second plurality of protrusions (raised features or components) outwardly extending toward and contacting the other of the separator plate and the cell layer at a plurality of contact points on the cell layer surrounding the inlet and the outlet for the flow of fluid to and from the first fluid volume. The electrochemical cell unit may further comprise a flanged perimeter on at least one of the separator plate and the cell layer. The flanged perimeter may be attached to the separator plate and the cell layer or may be integrally formed with the separator plate and the cell layer by pressing the plate and/or the cell layer. The flanged perimeter may be used to conjoin the separator plate and the cell layer together. For example, the separator plate and the cell layer may be directly adjoined at the flanged perimeter to form the first fluid volume therebetween. The flanged parameter of the separator plate and the cell layer may optionally be welded together, or adjoined directly through some other means.

The electrochemical cell unit may alternatively comprise a spacer plate provided and sandwiched between the separator plate and the metal support plate. The spacer plate may provide a separation between the metal support plate and the separator plate. For example, the spacer plate may be provided and sandwiched between the separator plate and the metal support plate to form the first fluid volume therebetween. Those three plates may be sealingly fixed to one another, for example by welding around their periphery.

The separator plate of the electrochemical cell unit may be configured or otherwise adapted to be exposed to a pressure at the first side of the separator plate that is less than a pressure at the second side of the separator plate. In other words, the separator plate may be configured to be able to survive a dual-pressure environment without becoming irrevocably damaged or distorted. The dual-pressure environment may be supplied to the separator plate by providing fluids of different pressures on the different sides of the separator plate to provide a pressure difference therebetween. When in-situ in a stack of cell unit(s), the separator plate(s) may be configured or otherwise adapted so that in the presence of a pressure difference between its first side and its second side, the first fluid volume formed between the separator plate and the cell layer is maintained.

For example, the pressure of a first fluid on the second side of the separator plate may be greater than that of a second fluid on the first side of the separator plate - the first fluid being a fuel for example, and the second fluid being an oxidant for example.

The pressure difference between the first side and the second side of the separator plate may be controlled by any number of means known to the person skilled in the art. For example, the pressure difference may be established through the use of pumps to pump the fluids at different rates and pressures. Alternatively, or in addition, features such as valves and chokes may be provided in the pipes or flow paths of the first fluid and the second fluid respectively to control the pressure difference between them. The pressure difference between them may be in the range of 50 mbar to 2 bar, preferably between 100 mbar to 1.5 bar, more preferably 200 mbar to 800 mbar. The skilled person will appreciate that the pressure difference used may be tailored to maintain the spacing between the separator plate and the cell layer, and said pressure difference may be dependent on the flexibility of the cell layer and the separator plate (the metal sheet thereof).

By providing features in the flow paths of the first fluid and the second fluid, the initial pressure at the inlet of the first fluid volume and the second fluid volume may be controlled to control the pressure difference between the first fluid volume and the second fluid volume (for fuel cell operation, for electrolysis cell operation only a first fluid may be provided and its initial pressure controlled). Additionally or alternatively, the pressure at the respective outlets of the first fluid volume and second fluid volume may be controlled to provide a pressure difference between the first fluid volume and second fluid volume.

The separator plate may also be configured or otherwise adapted to flex in when it experiences a pressure difference between its first side and its second side. In other words, the pressure difference experienced by the separator plate may be such as to cause the separator plate to flex away from, or toward the cell layer. Preferably, the separator plate may be configured to flex away from the cell layer when exposed to the pressure difference as a positive function of the pressure difference. By flexing away from the cell layer, the spaced arrangement between the separator and the cell layer may be maintained (or increased), and in turn a fluid volume therebetween may be maintained (the height thereof maintained or increased), and contact with an adjacent neighbouring cell unit may be improved.

A cell stack is provided, comprising a plurality of cell units as described above, wherein the second side (e.g. upper side) of the separator plate of a first cell unit faces the first side (e.g. lower side) of a cell layer of the first cell unit in a spaced arrangement to form a first fluid volume for first fluid therebetween, and the first side (e.g. lower side) of the separator plate of the first cell unit faces an electrochemically active cell area of a second, neighbouring, cell unit in the stack of cell units and defines a second fluid volume therebetween.

A cell stack is provided comprising: a plurality of cell units, each cell unit comprising: a cell layer comprising an electrochemically active cell area, the cell layer having a first side and a second side, the second side carrying the electrochemically active cell area; and a separator plate having a first side and a second side, the separator plate comprising a metal sheet, the second (e.g. upper) side of the separator plate underlying and facing the first (e.g. lower) side of the cell layer in a spaced arrangement to form a first fluid volume for first fluid therebetween. The first side of the separator plate extends across and faces an electrochemically active cell area of an adjacent cell unit in the cell stack in a spaced arrangement to form a second fluid volume for second fluid therebetween. The separator plate has a region that extends across at least the electrochemically active cell area, wherein the region is clear of protrusions directed toward the cell layer, or other components to separate the separator plate from the metal support plate. The separator plate is adapted to be exposed to a pressure difference between the first side and the second side of the separator plate to maintain the spaced arrangement that forms the first fluid volume.

The cell stack is configured such that, in operation as a fuel cell, the first fluid volume is for fuel and the second fluid volume is for oxidant. For example, the fuel may be a hydrogen-rich reformate stream (e.g. converted from a hydrocarbon fuel stream such as natural gas). The oxidant may be air or oxygen. In operation as an electrolysis cell, the first fluid volume is for steam.

The cell stack may also be configured such that the first side of the separator plate contacts an outermost layer of the electrochemically active cell area of an adjacent cell unit, providing electrical contact therebetween and having a contact resistance that decreases as the pressure difference between the first side and the second side of the separator plate increases. For example, as described above the pressure difference experienced by the separator plate may be provided by a pressure difference between a pressure of the first fluid in the first fluid volume and a pressure of an oxidant such as air or oxygen on an opposite side of the separator plate i.e. the second fluid volume.

In other words, by introducing a pressure difference either side of the separator plate in one cell unit, the separator plate may be forced to flex toward the electrochemically active area of an adjacent neighbouring cell unit thereby forcing the downward protrusions (or dimples) of the separator plate to come into contact with the electrochemically active area of an adjacent neighbouring cell unit. This flexing can be achieved across the entire active region without extensive protrusions on the other side of the separator plate that would otherwise provide a force in that direction. By bringing the downward protrusions into contact with the electrochemically active area of an adjacent neighbouring cell unit the contact resistance decreases, i.e. conductivity through the stack is improved.

In this manner, the separator plate is adapted to be exposed to a pressure difference between the first side and the second side of the separator plate to maintain the spaced arrangement that forms the first fluid volume. (To the extent that there may be protrusions on the second side of the separator plate (over a minority of the area thereof, e.g. 10% or 20%), such protrusions may be caused to separate under pressure from the first side of the metal support plate, lifting away from the holes/pores therein and permitting fuel access to the porous region of the support plate. Note also that, even if a few protrusions are provided on the second side of the separator plate, they are fewer in number than on the first side. Fluid pressure will give even pressure across the plate, obviating the need for protrusions on the second side across the entirety of the active region).

A method for manufacturing a cell unit is provided. The method comprises: providing a planar metal sheet for a separator plate having a first side and a second side, the planar metal sheet having protrusions; providing a cell layer comprising an electrochemically active cell area, the cell layer having a first side and a second side; and overlying the separator plate and the cell layer such that the separator plate faces the first side of the cell layer in a spaced arrangement to form a first fluid volume therebetween and the separator plate has a region that extends across at least the electrochemically active cell area. The region is clear of protrusions directed toward the cell layer or other components to separate the separator plate from the cell layer.

The method may comprise providing a metal support plate with a cell layer comprising an electrochemically active cell area, wherein the first side of the cell layer is a first side of a metal support plate and the second side of the cell layer is a second side of the metal support plate opposite the first side of the metal support plate, the second side carrying the electrochemically active cell area. The method may comprise pressing the planar metal sheet to provide the planar protrusions extending from the surface of the planar metal sheet

At least one of the separator plate and the cell layer (or the metal support plate supporting a cell layer) may be processed to form a flanged perimeter. The flanged perimeter of the separator plate and/or the cell layer (or the metal support plate supporting a cell layer) may be integrally formed with the separator plate and/or the cell layer (or the metal support plate supporting a cell layer) by pressing. During manufacture, the separator plate and the cell layer may be directly adjoined at the flanged perimeter to form the first fluid volume therebetween, optionally by welding.

As an alternative to a flanged perimeter (or in addition), a spacer plate may be provided between the separator plate and the metal support plate. The spacer plate may extend around the perimeter of the separator plate and/or the cell layer. It may serve to space the plates apart and define the first fluid volume. A method for manufacturing a cell unit stack is provided. The method comprises: providing a plurality of cell units, each cell unit manufactured as described above and overlaying/underlying one of the plurality of cell units with another one of the plurality of cell units such that the protrusions of the separator plate of the one of the plurality of cell units extend and come into contact with an electrochemically active cell area of another one of the plurality of cell units. The overlaying/underlying further comprises providing gaskets between the one of the plurality of cell units and the other one of the plurality of cell units.

A method of operating a cell stack of cell units is provided. The cell stack is as described above and the method comprises: providing a first fluid to the first fluid volume; providing a second fluid to the second fluid volume; and regulating a pressure difference between the first fluid volume and the second fluid volume to maintain the spaced arrangement that forms the first fluid volume.

In one aspect of the present invention there is provided a method of operating a cell stack of cell units is provided. In the method, each cell unit in the cell stack comprises: a cell layer comprising an electrochemically active cell area, the cell layer having a first side and a second side; a separator plate electrically connected to the cell layer, the separator plate having a first side and a second side, the second side of the separator plate extending across and facing the first side of the cell layer in a spaced arrangement to form a first fluid volume and, the first side of the separator plate comprising protrusions directed away from the first side of the cell layer and towards a second side of a cell layer of a neighbouring cell unit to form a second fluid volume. The method comprises: providing a first fluid to the first fluid volume; providing a second fluid to the second fluid volume; and regulating a pressure difference between the first fluid volume and the second fluid volume to maintain a spaced arrangement that forms the first fluid volume. In such a way, an electrical connection between the protrusions and the second side of the cell layer of the neighbouring cell unit can be controlled by the pressure difference.

In another aspect of the invention there is provided an electrochemical cell unit comprising: a cell layer comprising an electrochemically active cell area, the cell layer having a first side and a second side; a separator plate electrically connected to the cell layer, the separator plate having a first side and a second side, the second side of the separator plate extending across and facing the first side of the cell layer in a spaced arrangement to form a first fluid volume and, the first side of the separator plate comprising protrusions directed away from the first side of the cell layer and towards a second side of a cell layer of a neighbouring cell unit, wherein the separator plate is adapted to be exposed to a pressure difference between the first side and the second side of the separator plate to maintain the spaced arrangement that forms the first fluid volume and to bias the protrusions towards the second side of the cell layer of the neighbouring cell unit. In such a way, an electrical connection between the protrusions and the second side of the cell layer of the neighbouring cell unit can be controlled by the pressure difference.

In another aspect of the invention there is provided a method for manufacturing a cell stack comprising a plurality of electrochemical cell units, comprising: providing a plurality of cell units, each cell unit comprising a cell layer comprising an electrochemically active cell area, the cell layer having a first side and a second side, and a separator plate electrically connected to the cell layer, the separator plate having a first side and a second side, the second side of the separator plate extending across and facing the first side of the cell layer in a spaced arrangement to form a first fluid volume, the first side of the separator plate comprising protrusions directed away from the first side of the cell layer; and overlaying the plurality of cell units one upon another so that the first side of the separator plate of a first cell unit faces the second side of a second, neighbouring, cell unit in the stack of cell units and defines a second fluid volume therebetween, wherein said protrusions in the separator plate of the first cell unit are directed towards a second side of a cell layer of a neighbouring cell unit, and wherein the separator plate of the first cell unit is adapted to be exposed to a pressure difference between the first side and the second side of the separator plate to maintain the spaced arrangement that forms the first fluid volume and to bias the protrusions towards the second side of the cell layer of the neighbouring cell unit

Note that where the cell units are electrolysis cell units, the second fluid is generated in the reaction. In other words, the steps of providing encompass providing (from a source external to the cell units) initial reactant and providing (or generating, by the cell units) product of the electrochemical reaction at the cell units. For example, in electrolysis cell operation the first fluid volume of the cell units is provided with fuel (in the form of steam from a source external to the cell units) and a product of the electrolysis reaction, that product being hydrogen if the electrolyte is oxygen ion conducting or oxygen if the electrolyte is hydrogen ion conducting. Correspondingly, the second fluid volume of the cell units may only be provided with a product of the electrolysis reaction, that product being (in the example of steam as a fuel) oxygen if the electrolyte is oxygen ion conducting or hydrogen if the electrolyte is hydrogen ion conducting. A sweep gas (e.g., oxygen or air) may optionally be provided to the second fluid volume from a source external to the cell units. Such sweep gas may assist in exhausting product of the electrolysis reaction from the second fluid volume.

The methods of operating the cell stack may comprise: providing a fuel (e.g., a reformed hydrocarbon fuel or hydrogen in fuel cell operation, or steam in electrolysis cell operation, and product of said reaction) to a fuel volume of the each cell unit of the cell stack, the fuel volume of each cell unit formed between a respective separator plate and respective cell layer of each cell unit; providing air or oxygen (from a source external to the cell units in fuel cell operation, and as a product of the reaction or as a sweep gas in electrolysis cell operation) to an oxidant fluid volume of the each cell unit of the cell stack, the oxidant fluid volume of each cell unit formed between cell units of the cell stack; and regulating a pressure difference between the fuel volume and the oxidant volume by regulating the pressure of the reformed hydrocarbon fuel and the pressure of the air or oxygen respectively.

The pressure difference between the first fluid volume and the second fluid volume may be regulated to be in the range of 50 mbar to 2 bar, preferably between 100 mbar to 1.5 bar, more preferably 200 mbar to 800 mbar.

The pressure difference is preferably regulated to decrease an electrical contact resistance between the separator plate and the electrochemically active cell area of the second, neighbouring, cell unit in the stack of cell units. By increasing the pressure difference, the electrical contact resistance may be decreased, thereby improving efficiency of the stack.

The pressure difference may be regulated by, for example, i) the use of pumps for the pumping of the first fluid and the second fluid at different rates ii) chokes such as valves, or convergent- divergent nozzles (such as a de Laval nozzles) in a pipe or flow path providing the fluids to a first fluid inlet and a second fluid inlet of the cell units and/or the cell stack as a whole, iii) orifice plates in the pipe or flow path to assist in the regulation of the pressure difference between the first fluid and the second fluid. Other ways and apparatus that may be used to establish a pressure difference between the first fluid and the second fluid will be readily known by the person skilled in the art.

Brief Description of the Drawings

Figure 1 is an exploded perspective view of a fuel cell unit and two gaskets;

Figure 2 is a second perspective view of the arrangement in Figure 1, shown from a different angle; Figure 3 is a first exploded perspective view of a first arrangement comprising a stack of two cell units separated by gaskets, each cell with two fluid ports.

Figure 4 is an underside exploded perspective view of the arrangement in Figure 3;

Figure 5 is a cross sectional view of the arrangement in Figure 3;

Figure 6 is a first exploded perspective view of a second arrangement comprising a stack of two cell units separated by gaskets, each cell with four fluid ports;

Figure 7 is an underside exploded perspective view of the arrangement in Figure 6;

Figure 8 is a first exploded perspective view of a third arrangement comprising a stack of two cell units separated by gaskets, each cell with four fluid ports and a spacer plate.

Figure 9 is an underside exploded perspective view of the arrangement in Figure 8;

Figure 10 is a cross sectional view of the arrangement in Figure 8;

Figure 11 illustrates a method of manufacturing a cell unit in accordance with the present invention.

Figure 12 illustrates a method of operating a cell stack in a steady state in accordance with the present invention.

Detailed Description

For illustrative purposes only, the figures only indicate two electrochemical cell units (each hereafter referred to simply as a "cell unit") in a stack. In various embodiments, multiple cells are provided. In further embodiments (not shown) multiple electrochemical cell stacks are provided, and in still further embodiments multiple electrochemical cell stacks each comprising multiple electrochemical cells are provided. It will be appreciated that the anode and cathode inlets, outlets (off-gas), ducting, and manifolding, and their configuration are modified as appropriate for such embodiments, and will be readily apparent to a person of ordinary skill in the art.

Referring to Figure 3, cell unit 300 comprises a flat (i.e. planar) metal support plate 314 stacked next to a separator plate 312. The metal support plate 314 is shown to have flanged perimeter feature 318 around its perimeter. The flanged perimeter 318 extends out of the predominant plane of the sheet, as found at a central fluid volume area, to create a concavity in the metal support plate 314 (and a convexity to the outside surface). The concavity will form a first fluid volume 360 within this cell unit upon assembly of the cell unit. The separator plate has a first side and a second side, and comprises a metal sheet. The second side of the separator plate extends across and faces a first side of the cell layer. The two plates are sealed around their periphery (e.g. welded), to enclose/seal the enclosed first fluid volume.

The cell unit 300 has rounded ends and parallel sides, with one fluid port 322 towards each end in both the separator plate 312 and the metal support plate 314. Other shapes and sizes and numbers of the respective cell features are possible depending upon the required power and dimensions of the final stack assembly.

In a middle portion of the cell unit 300, an electrochemically active layer 350 is provided on a cell layer (here a metal support plate with a cell layer is shown). In this embodiment it is located outside of the first fluid volume 360.

The electrochemically active area 350 includes an anode, a cathode, and an electrolyte positioned between the anode and cathode (not shown). The anode, electrolyte, and cathode may together be referred to as the electrochemically active layer 350, active electrochemical cell layer, or electrochemically active region. The electrochemically active region may be a continuous and generally rectangular region, which may be generally uninterrupted. Alternatively, the electrochemically active cell region may be wrapped around the fluid ports to increase the proportion of the cell unit area that is electrochemically active and thereby increase a power density of the stack of cell units. In other words, in the vicinity of the ports, the edge of the active cell region is shaped to match the shape of the port. The edge of the active cell region forms a part-circle which is concentric with the port. The edge of the active cell region is spaced from the edge of the port to allow space for formed port features and/or gaskets disposed around the port.

The electrolyte conducts either negative oxygen ions or positive hydrogen ions between the anode and cathode.

The stack may comprise a stack of cell units that are based on one of solid oxide electrolytes, polymer electrolyte membranes, or molten electrolytes or any other variant capable of electrochemistry.

Figure 4, shows the same cell unit 300 as Figure 3 but from a different perspective view. Figure 5 shows a cross sectional view of Figure 3. The section is taken from rear-left to front-right, to the rear of centre. Around the fluid ports of the metal support 314, shaped port features 324 are provided. The shaped port features 324 are provided as multiple elements in the form of protrusions extending out of the plane of the base of the fluid volume a distance corresponding to that of the height of the flanged perimeter 318 - to have a common height therewith. This is so that they will contact the opposing surface of the separator plate 312, just like the flanged perimeter 318, when the cell unit 300 is assembled. As a result, when the flanged perimeter 318 are joined to the separator plate 312, for example by welding, the shaped port features 324 will likewise contact the separator plate 312. The protrusions may have a circular, square, cross, pentagonal, or hexagonalshaped cross-section. They may also be oval or irregular polygon in cross-section.

The concave configuration can give the relevant plate the appearance of a rimmed tray, with a correspondingly convex outside shape (outside relative to the cell unit) and usually a planar base, the concavity thus defining (e.g. part of) the first fluid volume 360 in the assembled cell unit. In this concave configuration, the flanged perimeter 318 extends out of a plane of the original sheet of the separator plate, and/or of the metal support plate, toward a respective opposed surface of the other of the separator plate and the metal support plate.

The first fluid volume 360 is thus bordered by a flanged perimeter 318, which is formed by pressing, such as by use of a die press, hydroforming or stamping.

The metal support plate 314 (e.g. metal foil) is provided with multiple small holes or pores 348 to enable first fluid in the first fluid volume 360 to be in fluidic communication with the electrochemical layers supported by a second side (upper side as shown) of the cell layer/metal support plate. These holes or pores form a porous region bounded by a non-porous region. The anode (fuel electrode) layer is located adjacent the small holes/pores with the (enclosed) fluid volume 360 within the cell unit comprising a first fluid volume 360 supplied by first fluid entering and exiting via the fluid ports 322.

The first fluid may be fuel ((reformed) hydrocarbon or other fuel (e.g. ammonia) when operated as a fuel cell, and steam when operated as an electrolysis cell) in such case the fluid ports 322 are thus fuel ports 322. The anode (fuel electrode) layer may be coated or otherwise deposited on the metal support plate 314. The cathode (air electrode) layer is on the opposite side of electrochemically active layer 350, i.e. on its outer face, and is exposed to air flowing across that layer during use of the cell unit. In the cell units depicted in Figure 3 & 4 & 5 only two layers (components) are required, i.e. the metal support plate and the separator plate.

The separator plate 312 is also provided with protrusions 336 which extend from the separator plate 312 towards an adjacent cell unit (in other words, away from the metal support plate 314 of the cell unit of which the separator plate is a constituent). Those downward protrusions 336 (within the plan view area of the electrochemically active layer) which include outward (down as shown) protrusions, extend from the separator plate 312 to contact, in a stack of cell units, an outer surface of the electrochemically active layer of a cell unit adjacent to the separator plate. The central downward protrusions 336 define fluid pathways between them or in them for oxidant (such as air) through a second fluid volume 365 defined between the outer surface of the electrochemically active layer of the cell unit adjacent to the downward protrusions.

When formed into a stack of cell units, a second side of the separator plate of a first cell unit faces a first side of a cell layer of a first cell unit, in a spaced arrangement to form the first fluid volume for the first fluid therebetween, and the first side of the separator plate of the first cell unit faces an electrochemically active cell area of a second, neighbouring, cell unit in the stack of cell units and defines a second fluid volume therebetween. The first fluid volume is for first fluid (such as fuel in the form of a reformed hydrocarbon fuel or other fuel (e.g. ammonia) in fuel cell operation, or for steam in electrolysis cell operation), and the second fluid volume is for second fluid (such as oxidant in fuel cell operation, or generated oxygen in electrolysis cell operation). Between each cell unit in the stack there is provided two or more gaskets 334 (one surrounding each port, of which there may be more than 2) underneath each cell unit i.e. gaskets are positioned between adjacent cell units in the stack. There may be plural inlet ports and plural outlet ports.

Each gasket 334 (also referred to as a "seal") provides a primary sealing function and will preferably be compressible. The gaskets are subjected to compressive forces in the vicinity of the ports to achieve the sealing function. For example through means capable of applying a compressive force. The gaskets may be sized to cover all the shaped port features 324 of each fluid port 322 to prevent first fluid (such as fuel in the form of a (reformed) hydrocarbon fuel or other fuel (e.g. ammonia) in fuel cell operation or steam in electrolysis cell operation) that may be travelling through the fluid ports 322 from seeping between the outside of the cell unit 300 and the gasket 334, into the area external of the cell units, i.e. into the second fluid volume surrounding the cell units 300 (such as oxidant in fuel cell operation, or generated oxygen in electrolysis cell operation), or the fluid external of the fluid ports from seeping in the other direction - into the fluid ports. This can assist in preventing any mixing of the fluid inside the cell unit 300 and the fluid outside the cell unit 300, which may be fuel and oxidant - the polarity of the electrochemically active layers 350 determining which way round this will be.

The gaskets may also provide electrical insulation between a first cell unit and an adjacent fluid cell unit, so as to prevent a short circuit. The gaskets may be any suitable cell gaskets (sealing rings), such as, for example, Thermiculite (trade mark).

Compressive forces in the stack in the vicinity of the electrochemically active layer are typically required for good electrical contact between cell units in the stack and hence good conductivity through the stack. The central downward protrusions 336 create the required electrical contacts between cell units (and the adjoining, preferable fixing by welding, of separator and cell layer mean that those components are electrically connected). For example, when in a stack arrangement, the central downward protrusions 336 on the first side of the separator plate 312 of a first cell unit in the stack contacts an outermost layer of the electrochemically active cell area of an adjacent cell unit in the stack, providing electrical contact therebetween.

Unlike in the prior art shown in Figures 1 & 2, the first arrangement does not have central protrusions i.e. protrusions extending between the internal opposed surfaces of the two plates (i.e. the separator plate and the cell layer/metal support plate). Accordingly, the fuel is able to enter the first fluid volume 360 through the fluid ports 322, and flow freely across the whole surface of the separator plate 312 and through the whole first fluid volume.

Furthermore, as shown in Figures 3 & 4 & 5, the region of the separator plate 312 that underlies the electrochemically active layers 350 of the cell is clear of any other components that may act to separate the separator plate 312 from the metal support plate 314.

Also, unlike in the prior art shown in Figures 1 & 2, as the first arrangement does not have central upward protrusions extending between the internal opposed surfaces of the two plates, there is an absence of a feature in the cell unit that provides a support function for the cell unit in the central region, extending towards (or, as shown in Figures 3 & 4 & 5, upwardly to the underside of) the metal support plate at the area of the small holes (also referred to as the porous region, and corresponding to the plan view extent of the electrochemically active layers).

In previous designs (e.g. Figures 1 and 2), the cell units are stacked with the gaskets 34 between each repeat unit. Before compression, the gaskets 34 are thicker than the height of the protrusions 36 and the protrusions are not in contact with the next unit. As the stack is compressed, initially the compressive force acts solely through the gaskets (since the protrusions are not in contact). At a certain point, the gaskets 34 will be sufficiently compressed that the protrusions then come into contact. As the stack is further compressed, the compressive force acts through the gaskets 34 and the protrusions 36. This can lead to problems as described above.

With the new concept, since there are no protrusions directed toward the cell layer 348, the gaskets 334 can be compressed as required, without the compressive force also acting through protrusions or other structures in the vicinity of the active cell area (there may be some movement of the substrate/interconnect depending on stiffness, etc of the plates, but such movement is minor). Thus the compression in the active cell area is decoupled from the gasket compression, and can be controlled by the pressure difference - such pressure difference acting to push the cell layer (metal support plate 314 thereof) and interconnect 312 of that cell unit (of which said cell layer and interconnect are constituents) apart. In turn, the interconnect of that cell unit is urged towards and in contact with a neighbouring cell unit (typically the electrochemically active layer of the cell layer of a neighbouring cell unit) and thereby producing the required electrical contact between neighbouring cell units. With the new arrangement, the force conveyed by the protrusions 316 may be greatly reduced. The final force through the protrusions and the active area is achieved by the pressure difference.

Referring to Figures 6 and 7, the cell unit 600 is similar to the cell unit 300 of Figures 3, 4 & 5 save that the separator plate 612 of the cell unit 600 is shown to have flanged perimeter 618 instead of the metal support plate 614, the shaped port features are provided in the separator plate 618, and a different arrangement of fluid ports is provided. The flanged perimeter 618 extends 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 a first fluid volume 660 within this cell unit upon assembly of the cell unit.

It should be noted that the arrangement of Figs. 6 and 7 can be modified in ways already discussed in relation to the arrangement of Figures 3, 4 and 5. For example: the first fluid volume may be formed by a flanged perimeter in either or both of the separator and the metal support plate. Where both have flanges, the shaped port features may have a total height the same as the sum of both flanges. There may be a flange in one plate and port features in the other. There may be more than two ports. The cell unit 600 has rounded ends and parallel sides, with one fluid port 622 towards each corner of both the separator plate 612 and the metal support plate 614, thereby giving a total of four fluid ports 622. Other shapes and sizes and numbers of the respective cell features are possible depending upon the required power and dimensions of the final stack assembly.

Around the fluid ports 622 of the separator plate 612, shaped port features are provided similar to the shaped port features of 324 of cell unit 300. The shaped port features are provided as multiple elements in the form of protrusions extending out of the plane of the base of the fluid volume 660 a distance corresponding to that of the height of the flanged perimeter 618 - to have a common height therewith. This is so that they will contact the opposing surface of the metal support plate 614, just like the flanged perimeter 618, when the cell unit 600 is assembled. As a result, when the flanged perimeter 618 is joined to the metal support plate 614, for example by welding, the shaped port features will likewise contact the metal support plate 614. The protrusions may have a circular, square, cross, pentagonal, or hexagonal-shaped cross-section. They may also be oval or irregular polygon in cross-section.

Referring to Figures 8, 9, and 10, the cell unit 800 is similar to the cell unit 300 & 600 described above (and is shown in similar views) save that neither the separator plate 812 nor the metal support plate 814 of the cell unit has a flanged perimeter. To create a first, enclosed, fluid volume 860 between the separator plate 812 and the metal support plate 814 of the cell unit 800, a spacer plate 816 is provided between the separator plate 812 and the metal support plate 814.

The cell unit 800 has rounded ends and parallel sides, with one fluid port 822 towards each corner of the separator plate 812, the metal support plate 814 and the spacer plate 816, thereby giving a total of four fluid ports 822. Other shapes and sizes and numbers of the respective cell features are possible depending upon the required power and dimensions of the final stack assembly.

The spacer plate 816, when in position in the cell unit overlies/underlies the perimeter of the separator plate 812 and underlies/overlies the perimeter of the metal support plate 814. A central hollow portion 817 of the spacer plate 816 at least overlies/underlies the central downward protrusions 836 extending between the separator plate 812 and the area of the electrochemically active layer of the cell unit adjacent to the outward protrusions. The hollow central portion 817 also at least underlies/overlies the porous region (multiple small holes) provided in the metal support plate 812 to enable fluid in the first fluid volume to be in fluidic communication with the side of the electrochemical layers that is closest to the metal support plate 814. The hollow portion 817 of the spacer plate 816, when sandwiched between the separator plate 812 and the metal support plate

814, forms a fluid volume between the separator plate 812 and the metal support plate 814 for fuel.

Unlike the first and second arrangements, the third arrangement does not have shaped port features around the fluid ports of the separator plate. The spacer plate acts to provide a separation between the metal support plate and the separator plate of the cell unit. Throats in the spacer plate allow fluidic communication between the ports and the first fluid volume.

In each arrangement described above, as there are no central upward protrusions extending between the internal opposed surfaces of the two plates (i.e. the separator plate and the cell layer/metal support plate), there is provided means of establishing and maintaining the first fluid volume 360; 660; 860 between the metal support plate 314; 614; 814 and the separator plate 312; 612; 812, during operation of the cell unit.

Where the cell unit 300; 600; 800 is a fuel cell unit (or a stack of fuel cell units), a fuel (i.e. the anode inlet gas eg., a hydrocarbon fuel, reformed hydrocarbon fuel, H2, ammonia) is passed to the anode inlet of the cell unit and enters the first fluid volume (fuel volume) between the separator plate 312; 612; 812 and the cell layer (or metal support plate 314; 614; 814), via the ports 332; 632; 832. At the same time oxidant (i.e. the cathode inlet gas) is passed to the cathode inlet of the cell unit to flow either side of separator plate 312; 612; 812 and the cell layer (or metal support plate 314; 614; 814). The fuel and oxidant may flow in a co-flow configuration such that the fuel and the oxidant flow in the same direction across their respective sides of the cell unit. Alternatively, the fuel and oxidant may flow in counter or cross flow configuration.

Where the cell unit 300; 600; 800 is a fuel cell unit, the fuel and the oxidant are provided to the fuel cell unit at different pressures to provide a pressure difference between the fuel and the oxidant as they pass through the fuel cell unit. That in turn results in there being a pressure difference between a first side (proximate to the oxidant) and a second side of the separator plate 312; 612; 812 (proximate to the fuel). By providing a pressure difference between the first side and the second side, a separation between (the second side of) the separator plate and (the first side of) the cell layer (or the first side of the metal support plate 314; 614; 814) can be controlled. For example the separation can be maintained or increased to create and maintain the first fluid volume.

To enable the separation between the separator plate 312; 612; 812 and the cell layer (or metal support plate 314; 614; 814) to be maintained or increased through the provision of a pressure difference between the first side and the second side, the separator plate may be adapted, or configured, to flex when exposed to the pressure difference. For example, when exposed to the pressure difference the separator plate may flex away from the cell layer (or metal support plate) of the cell unit (and toward a neighboring cell unit) as the pressure difference is increased i.e. the separator plate is adapted to flex away from the cell layer (or metal support plate) when exposed to the pressure difference as a positive function of the pressure difference.

When in a stack arrangement, the central downward protrusions 336; 636; 836 on the first side of the separator plate 312; 612; 812 of a first cell unit in the stack contact an outermost layer of the electrochemically active cell area of an adjacent cell unit in the stack, providing electrical contact therebetween.

When each separator plate 312; 612; 812 is exposed to a pressure difference so as to cause it to flex away from the cell layer (or metal support plate 314; 614; 814) of its corresponding cell unit, a contact resistance between the central downward protrusions 336; 636; 836 and an outermost layer of the electrochemically active cell area of an adjacent cell unit decreases. In other words, the contact resistance between the central downward protrusions 336; 636; 836 and an outermost layer of the electrochemically active cell area of an adjacent cell unit decreases as the pressure difference between the first side and the second side of the separator plate increases.

During operation the pressure difference the first side and the second side of the separator plate can be controlled to be in the range of 50 mbar to 2 bar, preferably between 100 mbar to 1.5 bar, more preferably 200 mbar to 800 mbar.

A method of manufacturing any of the cell units described in any of the above embodiments includes a number of steps/operations. That method includes the following steps.

At step 1110, a separator plate having a first side and a second side is provided e.g. by cutting or stamping. The separator plate may, for example, be a planar metal sheet that is non-porous, or any other planar sheet that is non-porous, and which acts to separate one cell unit from an adjacent cell unit in a stack. The separator plate may be provided with protrusions extending out of a plate of the separator plate, which may be provided by pressing/forming in the same step as the cutting/stamping.

At step 1120, a cell layer is provided comprising an electrochemically active cell area which includes an anode, a cathode, and an electrolyte positioned between the anode and cathode (not shown). The cell layer has a first side and a second side, and may preferably be a metal supported cell layer. Adding the cell layer may include depositing or coating the cell layer on a planar metal sheet e.g. by printing the electrochemically active cell area on the cell layer, thus forming a metal supported cell layer, with a porous region (holes) providing fluidic communication from the first side to the electrode supported by the metal support plate on its second side. Alternatively, the cell layer may support itself. For example, the cell layer has an anode-supported, electrolyte-supported, or cathode-supported architecture. For the purposes of illustration only, the term 'metal support plate' is used in the following passages, but can be interchanged with 'cell layer' or 'metal plate supported cell layer'.

Step 1110 or step 1120 preferably involves either providing a cell layer (or the metal plate supported cell layer) that has a flanged perimeter 318, or a separator plate that has a flanged perimeter 618. The flanged perimeter 318 or 618 extends out of the predominant plane of the metal plate support plate 314 or the separator plate 618 respectively.

The flanged perimeter 318 creates a concavity in the metal support plate 314 (and a convexity to the outside surface). The concavity forms a first fluid volume 360 within this cell unit upon assembly of the cell unit.

The flanged perimeter 618 creates a concavity in the separator plate (and a convexity to the outside surface). The concavity forms a fluid volume within this cell unit upon assembly of the cell unit.

The flanged perimeter in either the separator plate or the metal support plate may be made by pressing the separator plate or the metal support plate (of the cell layer) respectively.

Alternative to the flanged perimeter, a spacer plate may be provided and sandwiched between separator plate and metal support plate to form the first fluid volume therebetween.

Step 1110 and step 1120 also include providing a plurality of fluid ports 322; 622; 822 in both of the separator plate and the metal support plate to allow for the flow of a fluid (such as reformed fuel) through the cell units (and ultimately through a stack of cell units) to provide fuel to each cell unit, in particular to provide fuel to the first fluid volume of each cell unit.

At step 1130, the separator plate and the metal support plate are overlaid in a spaced arrangement to form a first fluid volume therebetween. Thus, the separator plate has a region that extends across at least the electrochemically active cell area. At step 1130, the separator plate and the metal support plate are overlaid so that the protrusions extending out of a plane of the separator plate are orientated to point away from the first fluid volume toward an adjacent cell unit when the cell unit is placed in a stack arrangement. In other words there is a continuous region that extends across at least the electrochemically active cell area that is clear of protrusions directed toward the metal support plate. It is also clear of any other component which is configured to resist a stack compression force and transfer such force to the protrusions which connect adjacent cell units. Thus there is no component within the first fluid volume, between the separator plate and metal support plate, to assist (in particular in operation) in the physical separation of them from one another.

At step 1130 the separator plate and metal support plate may be directly adjoined (and sealingly adjoined) at the flanged perimeter described above to form the first fluid volume therebetween. The separator plate and metal support plate may be directly adjoined optionally by welding.

In the alternative arrangement without the flanged perimeter, a spacer plate being provided and sandwiched between separator plate and metal support plate to form the first fluid volume therebetween, and at 1130 those three plates are sealingly fixed to one another, for example by welding around their periphery.

When forming a stack of cell units the method may continue, wherein the second side of the separator plate of a first cell unit (formed as described above) is arranged to overlie/underlie a second cell unit such that the first side of the separator plate of the first cell unit faces an electrochemically active cell area of a second, neighbouring, cell unit in the stack of cell units and encloses a second fluid volume therebetween. When forming the stack, a plurality of gaskets is provided, corresponding to the plurality of fluid ports of the cell units. Each gasket is positioned around the fluid ports of adjacent cell units in the stack. The function of the gaskets is already described above.

A method of operating a cell stack of cell units as described in the above embodiments includes a number of steps/operations, as follows.

At step 1210, a first fluid is provided to the first fluid volume formed between the separator plate and metal support plate. The first fluid may be a fuel. For operation as a fuel cell, the first fluid may be a hydrocarbon fuel, a reformed hydrocarbon fuel, ammonia, H2, methanol, etc. For operation as an electrolysis cell, the first fluid is typically steam.

At step 1220, a second fluid is provided to the second fluid volume formed between the separator plate of a first cell unit in the stack and an electrochemically active cell area of a second, neighbouring, cell unit in the stack. The second fluid may be an oxidant fluid. For operation as a fuel cell, the second fluid may be an oxidant fluid, for example, air or oxygen provided to the second fluid volume via an inlet. For operation as an electrolysis cell, the second fluid is typically oxygen produced in the electrolysis reaction. At step 1230 a pressure difference between the first fluid volume and the second fluid volume is regulated to maintain the spaced arrangement that forms the first fluid volume. For example, a pressure of the first fluid (such as fuel) and the second fluid (such as air/oxygen) may be adjusted to produce a pressure difference between the two fluids. That pressure difference in turn may cause the separator plate to flex and a separation between the separator plate and the metal support plate may increase to form and maintain the spaced arrangement that forms the first fluid volume. The pressure difference between the first fluid volume and the second fluid volume may be in the range of 50 mbar to 2 bar, preferably between 100 mbar to 1.5 bar, more preferably 200 mbar to 800 mbar. Regulation of the pressure difference may also be to decrease an electrical contact resistance between the separator plate and the electrochemically active cell area of the second, neighbouring, cell unit in the stack (pressure in first fluid volume controlled to be greater than that in the second fluid volume, as that pressure difference is increased, contact resistance decreases).

The pressure difference may be regulated through the use of pressure pumps for the pumping of the first fluid and the second fluid at different rates. Alternatively, or additionally the flow of the first and/or second fluids may be chocked through the provision of a valve, or a convergent-divergent nozzle (such as a de Laval nozzle) in a pipe or flow path providing the fluids to the cell units of the stack. Alternative, or additionally, an orifice plate may be provided in a pipe or flow path to assist in the regulation of the pressure difference. Other ways and apparatus that may be used to establish a pressure difference will be readily known by the person skilled in the art.

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.

These and other features of the present invention have been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims.