WO2013156075A1

WO2013156075A1 - Metallic element useful as spacer or interconnect for a high temperature fuel cell

Info

Publication number: WO2013156075A1
Application number: PCT/EP2012/057244
Authority: WO
Inventors: Harald Usterud; Ulrik Jens NIELSEN; Christian Olsen
Original assignee: Topsøe Fuel Cell A/S
Priority date: 2012-04-20
Filing date: 2012-04-20
Publication date: 2013-10-24

Abstract

The invention concerns ametallic element for a high tem- perature fuel cell stack, wherein each individual fuel cell comprises an anode, a cathode, an electrolyte and an elec- trochemically active area. The metallic element (1) com- prises a metallic sheet (2) having first and second planar surfaces (3,4) on opposite sides of the metallic sheet (2) and a first, second, third and fourth periphery (5,6,7,8), the first and third peripheries (5,7) and the second and fourth peripheries (6,8) forming opposing peripheries. The metallic sheet (1) is provided with inlet and outlet gas openings (99,100) extending completely through the metallic sheet from the first planar surface (3) to the second pla- nar surface (4) and positioned at first and third peripher- ies (5,7) only of the metallic sheet (2), at least one inlet gas opening (99) and at least one outlet gas (100) opening being partially open and extending into one or more micro channels (13) located on the first planar surface (3). Each micro channel (13) extends continuously or dis- continuously from the at least one inlet gas opening (99) or at least one outlet gas opening (100) adjacent to the electrochemically active area of the fuel cell. This metal- lic element ensures an improved flow distribution of gases taking part in electrochemical reactions within the fuel cell, and it is well suited for use as an interconnect in a solid oxide fuel cell (SOFC) or a solid oxide electrolysis cell (SOEC).

Description

METALLIC ELEMENT USEFUL AS SPACER OR INTERCONNECT FOR A HIGH

TEMPERATURE FUEL CELL

The invention concerns a metallic element for a high tem^¬ perature fuel cell such as a Solid Oxide Fuel Cell (SOFC) , a Molten Carbonate Fuel Cell (MCFC) or a Solid Oxide Elec^¬ trolysis Cell (SOEC) . In particular it concerns a metallic element for an SOFC, MCFC or SOEC useful for providing im^¬ proved flow distribution of gas to the components of the cell .

Solid oxide fuel cells and molten carbonate fuel cells are commonly known as high temperature fuel cells due to their high operation temperatures of about 600-1000°C. In con^¬ trast to other fuel cells such as polymer electrolyte fuel cells, which operate at lower temperatures of about 50- 180°C, solid oxide fuel cells and molten carbonate fuel cells are made from materials that can withstand these high temperatures, such as ceramics, metallic components, glasses and combinations thereof.

A solid oxide electrolysis cell (SOEC) is a solid oxide fuel cell set in regenerative mode for the electrolysis of water with a solid oxide electrolyte to produce oxygen and hydrogen gas. An SOEC operates at temperatures suitable for high-temperature electrolysis, i.e. temperatures similar to those of an SOFC.

An SOFC comprises an oxygen-ion conducting electrolyte, a cathode at which oxygen is reduced and an anode at which hydrogen is oxidised. The overall reaction in an SOFC is that hydrogen and oxygen react electrochemically to produce electricity, heat and water. The anode also comprises a high catalytic activity for the steam reforming of hydrocarbons into hydrogen, carbon dioxide and carbon monoxide, the so-called internal reforming. Steam reforming can be described by the reaction of a hydrocarbon fuel, such as natural gas or methane, with steam, and the reactions which take place can be represented by the following equations: CH₄ + H₂0 —► CO + 3H₂

CH₄ + C0₂ —► 2CO + 2H₂

CO + H₂0 —► C0₂ + H₂

During internal reforming the hydrocarbon fuel gas supplied to the fuel cell in most cases contains steam, which en^¬ ables the endothermic steam reforming process to occur ac^¬ cording to the above equations at the anode surface. The overall reaction results in the production of electricity, heat and water.

The gases taking part in the electrochemical reactions and the gaseous products flow in and out, respectively, of the fuel cells through a manifold formed by aligned perfora^¬ tions near the edges of the fuel cell components. Hydrogen flows from its inlet manifold across the anode in a first direction to its outlet manifold, and oxygen flows from its inlet manifolds across the cathode in a second direction to its outlet manifold. The anode and the cathode require a free space above their surface to allow the passage of gas across their surface from the inlet manifold to the outlet manifold. This free space is provided by a metallic spacer element having a thickness which determines the free space available above the anode and cathode for passage of gas. A metallic interconnect positioned between two adjacent fuel cells and provided with passageways for gas flow, keeps the oxygen flow separate from the flow of the hydro^¬ carbon fuel or hydrogen, and provides electrical connection between the adjacent fuel cells present in a fuel cell stack . It is important to have an effective flow of gases across the fuel cell in order to reduce or avoid temperature gra^¬ dients, and several methods of reducing the temperature gradients are known. Most of these methods involve changes in operation parameters of the fuel cell system such as providing enhanced airflow to the cathode. Such changes are often associated with increased operational costs of the fuel cell system.

A number of different spacers and interconnects for fuel cells and electrolysis cells are described in the prior art. For example, US patent no. 7.270.906 B2 discloses pla^¬ nar spacers suitable for both the cathode and the anode. These planar spacers are formed by stamping from sheet stock, and they have a thickness that determines the height of either the cathode air flow space or the anode fuel flow space. Each spacer has gas passages along its edges which are completely open in the plane of the spacer along two opposing edges to allow one of the two gases present to flow across either the anode or the cathode surface, or closed in the plane of the spacer along the other two op^¬ posing edges to prevent the flow of the second gas across the same surface as the first gas. This type of spacer has several disadvantages. Since all the four edges of the planar spacers have completely open gas passages, the risk of having leakages in the area of the gas passages is increased. Completely open gas passages along all four edges also utilise more surface area of the fuel cell, rendering less surface area available for the electrochemical reactions. This makes it difficult to con^¬ duct parallel flows (co/counter) which are more electro- chemically effective.

A number of other publications describe the construction and shaping of spacers and interconnects with various forms of dimples, bosses, pins or other protrusions. For example, an interconnect element with at least one surface including dimples, bosses and/or pins arranged in a two-dimensional pattern is described in US 2005/0194720 Al . Further, an etched interconnect with small diameter, closely spaced contact pins is described in US 2002/0004155 Al, and more- over, WO 2007/088551 A2 discloses a flow distributor plate with displaceable metallic tabs made from incisions in the plate to create apertures.

These conventional interconnects and spacers with com- pletely open gas passages alternating with closed gas pas^¬ sages furthermore have local areas with high current densi^¬ ties which indicate the possibility of high degradation in those areas. Additionally, between two completely open gas passages there are sections which experience a poor gas distribution, and these sections are less effective during electrochemical reactions. It is therefore an objective of the invention to provide a metallic element for a high temperature fuel cell which en^¬ sures an improved flow distribution of gases taking part in electrochemical reactions within the fuel cell.

It is a further objective of the invention to provide a me^¬ tallic element for a solid oxide fuel cell (SOFC) in which internal reforming takes place. Yet an objective of the invention is to provide a metallic element suitable for use as a spacer or an interconnect for use in a solid oxide electrolysis cell.

Another objective of the invention is to provide a metallic element suitable for use as a spacer or an interconnect for a solid oxide fuel cell stack.

The main objective of the invention is to even out the flow distribution within a high temperature fuel cell, and this objective is attained by using a metallic element provided with gas passages having micro channels etched on the sur^¬ face of a base plate for transfer of gas from the manifold to the components of the fuel cell stack. The gas is dis^¬ tributed within the fuel cell across the entire surface of the cell using a metallic element having gas passages along two opposing peripheries only. The gas passages are either partially open in the plane of the metallic element to al^¬ low a first gas to flow across the surface of the metallic element and simultaneously across the adjacent anode or cathode surface, or the gas passages are completely closed to prevent the flow of a second gas across either the cath^¬ ode or anode surface, respectively. When the gas passages are partially open, they are provided with micro channels through which the gas is transferred from the gas passage to the electrochemical surface where reaction takes place.

According to the invention there is therefore provided a metallic element for a fuel cell stack, each individual fuel cell comprising an anode, a cathode, an electrolyte and an electrochemically active area, the metallic element comprising a metallic sheet having first and second planar surfaces on opposite sides of the metallic sheet and a first, second, third and fourth periphery, the first and third peripheries and the second and fourth peripheries respectively forming opposing peripheries. The metallic sheet is provided with inlet and outlet gas passages ex^¬ tending completely through the metallic sheet from the first planar surface to the second planar surface and posi^¬ tioned at first and third peripheries only of the metallic sheet, at least one inlet gas passage and at least one out^¬ let gas passage being partially open and extending into one or more micro channels located on the first planar surface 3. Each micro channel is extending continuously or discon- tinuously from the at least one inlet gas opening or at least one outlet gas opening adjacent to the electrochemi^¬ cally active area of the fuel cell.

According to another aspect of the invention there is provided a spacer for a fuel cell, wherein the metallic sheet comprises a central free space adjacent to and having ap^¬ proximately the same surface area as the electrochemically active area. According to yet another aspect of the invention there is provided an interconnect for a fuel cell, wherein the me^¬ tallic sheet is provided with a plurality of dimples, rods or protrusions providing flow passages in the metallic sheet adjacent to the electrochemically active area of the fuel cell.

The invention also provides a fuel cell comprising an elec- trolyte, an anode, a cathode and the metallic element.

Furthermore, the invention provides a fuel cell stack comprising at least two fuel cells. The metallic element of the invention provides an even dis^¬ tribution of especially the fuel gas, resulting in a sig^¬ nificant improvement of the performance of the fuel cell stack compared to conventional spacers and/or intercon^¬ nects .

The metallic element of the invention allows either an in^¬ crease or a decrease in the amount of fuel or air available at the electrode by increasing or decreasing the number of gas passages or their size and simultaneously adjusting the size of the micro channels. This allows for the design of stacks with for example fewer gas passages and larger micro channels, thereby reducing pressure drop and leakage.

By the term "partially open" is meant a gas passage which is not intact throughout its height and which has a disrup^¬ tion of the circular geometry of the gas passage, the dis^¬ ruption extending only partially through the thickness of the metallic plate. The height of the gas passage corre^¬ sponds to the thickness of the metallic element.

By the term "completely open" is meant a gas passage which is not intact throughout its height and which has a disrup^¬ tion of its circular geometry of the gas opening, the disruption extending completely through the thickness of the metallic plate. By the term "closed" is meant a gas passage which is intact throughout its height and thus the geometry of the gas pas^¬ sage is not disrupted.

The following is a brief description of the Figs.:

Fig. 1 shows a conventional spacer,

Fig. 2 shows a metallic element of the invention, Fig. 3 shows a detail of the metallic element of Fig. 2.,

Fig. 4 shows a metallic element of the invention,

Fig. 5 shows the first planar surface of a metallic element of the invention,

Fig. 6 shows the second planar surface of the metallic ele^¬ ment in Fig. 5, and Fig. 7 shows a section with discontinuous channels on a surface of a metallic element of the invention. The invention concerns a metallic element for an SOFC or MCFC fuel cell useful to even out the flow distribution of gases to the components of the fuel cell. The metallic ele^¬ ment comprises a metallic sheet having first and second planar surfaces on opposite sides of the metallic sheet, and having four peripheries and at least one gas passage on first and third opposing peripheries, for transfer of gas to and from the manifold to the fuel cell. The other two opposing peripheries, that is, the second and fourth pe- ripheries, are not provided with any gas passages. This is advantageous because the section of the metallic sheet lo^¬ cated around the circumference of the sheet required for sealing can be minimised and thereby the electrochemically active section can be increased. The output of the fuel cell stack is thereby increased.

The metallic element is provided with micro channels ex^¬ tending from partially open gas passages to the electro^¬ chemically active section of the fuel cell. Thereby a pas- sageway for gas is established from the manifold across the metallic sheet to the electrodes of the fuel cell.

The gas passages extend completely through the metallic sheet of the metallic element from the first planar surface to the second planar surface. The gas passages that are partially open on the first planar surface of the metallic sheet can be closed on the second planar surface, and the gas passages that are closed on the first planar surface can be partially open on the second planar surface. The gas passages are therefore defined as being partially open since the disruption created in the passage extends only partially through the metallic sheet. A disruption in the partially open gas passage extends into one or more micro channels, each micro channel having di^¬ mensions of up to 10 mm. The partially open gas passage has at least one disruption, each disruption extending into one or more micro channels. The length, width and depth of the micro channels can be varied, allowing the design of a me^¬ tallic element with one or a few partially open gas pas^¬ sages extending into several micro channels of a larger size. This results in a smaller pressure drop across the fuel cell and less gas leakage.

The micro channels can be straight or curved. If the par^¬ tially open gas passage has at least two straight micro channels extending from the disruption in the gas opening, the surface area between the two straight micro channels forms a trapezoid. If the straight micro channels are par^¬ allel to one another, the trapezoid resembles a parallelo^¬ gram. If the straight micro channels extend radially from the partially open gas passage, the trapezoid has no paral^¬ lel sides.

The trapezoid-shaped surface area between two straight mi^¬ cro channels is important because it contacts the fuel cell elements above and below the metallic element, thus allow^¬ ing transmission of pressure throughout the assembled fuel cell stack during operation.

The length of the micro channels can extend across the whole surface of the metallic element to its borders. This improves the gas distribution across the fuel cell leading to an improved performance of the fuel cell stack during operation .

The metallic element of the invention is suitable as a spacer or an interconnect for solid oxide fuel cells or for molten carbonate fuel cells. Since only two of the four pe^¬ ripheries of the metallic element have gas passages, the possibilities of having leakages in the area of the gas openings are reduced. Gas passages being present along only two of the peripheries also take up less surface area of the fuel cell, rendering more surface area available for the electrochemical reactions, making the fuel cell more electrochemically effective. When the metallic element is intended to be used as a spacer, the central section of the metallic element corre^¬ sponding to the electrochemically active section is devoid of metallic material and is thus empty, forming a central free surface for passage of gas across the electrochemi- cally active section.

The peripheries are provided with one or more gas passages that are partially open and one or more gas passages that are closed in the same plane or surface of the metallic element. The gas passages can be partially opened by creat^¬ ing micro channels in the metallic sheet. These micro chan^¬ nels have a predetermined depth and can be created by e.g. etching . The gas passages are either partially open in the plane of the metallic element to allow gas to flow across the cen^¬ tral free space on the anode or the cathode surface, or closed to prevent the flow of gas across either the cathode or the anode surface, respectively. When the gas passages are partially open, they extend into micro channels, whereby a passageway for the gas is created, so that the gas leaves the manifold and is distributed to and across the free space of the electrode. When the gas passages are closed, no micro channels are present. Micro channels can be present on both the first and the second opposing sur^¬ faces of the metallic element.

Each gas passage, when partially open, has at least one mi^¬ cro channel. Each micro channel has a length L defined as the distance between the first and second open ends. The first open end is located at the disruption in the geometry of the partially open gas passage. The second open end op^¬ poses the first open end. The width W of each micro channel is defined as the distance between its metallic sides. The depth D of each micro channel is defined by the etching depth. Each micro channel can have the same width through- out its length, or the micro channel can be diverging, such that its width at the first open end located at the gas passage is smaller than its width at the second open end located for instance at the electrochemically active sec^¬ tion of the metallic element.

It is important that the flow passages provided by the mi^¬ cro channels both allow the gas to flow unhindered towards the second opposing periphery and allow simultaneous contact of the gas with the electrode across the whole elec- trochemically active section of the fuel cell. The micro channels can be prepared by etching, machining, pressing, laminating or casting the metallic element. Preferably they are prepared by etching or pressing, as a large number of units can be prepared by these methods.

The metallic element may have micro channels on both its opposing planar surfaces. The micro channels on the first planar surface can be identically positioned with the micro channels on the second planar surface, or they can be dis- placed or shifted in relation thereto. However, the metal^¬ lic element may also have micro channels on only one of its two opposing planar surfaces.

In one embodiment of the invention, the metallic element has micro channels on both opposing planar surfaces. How^¬ ever, the gas passages which are provided with micro chan^¬ nels on the first planar surface do not have any micro channels on the second planar surface. Likewise the gas passages which are not provided with micro channels on the first planar surface have micro channels in the gas pas^¬ sages on the second planar surface of the metallic element. This allows for cross flow of the gases across the electro- chemically active section of the solid oxide fuel cell. Cross flow means that the direction of gas flow on the first planar surface of the metallic element is perpendicu^¬ lar to the direction of gas flow on the second planar surface of the metallic element. In another embodiment of the invention the gas passages which are provided with micro channels on the first planar surface of the metallic element also have micro channels on the second planar surface of the metallic element. Likewise the gas passages which are not provided with micro channels on the first planar surface do not have micro channels in the gas passages on the second planar surface of the metal- lie element. This allows for co-flow of the gases across the electrochemically active section.

By co-flow is meant that the direction of gas flow on the first planar surface of the metallic element is similar or identical to the direction of gas flow on the second planar surface of the metallic element.

Counter flow means that the direction of gas flow on the first planar surface of the metallic element is opposite to the direction of gas flow on the second planar surface of the metallic element.

In a further embodiment of the invention, the metallic ele^¬ ment is formed as a single metallic plate suitable for use as an interconnect in a solid oxide fuel cell or a solid oxide electrolysis cell.

When the metallic element is suitable for use as an inter^¬ connect, the central section of the metallic element adja- cent to the electrochemically active section is not a cen^¬ tral free space. The metallic element, when suitable for use as an interconnect, can be formed from a single con^¬ tinuous metal sheet provided with partially open gas pas^¬ sages and micro channels as described earlier and a plural- ity of dimples, rods, protrusions or small diameter, closely spaced contact points providing flow passages in the electrochemically active area of the fuel cell. The flow passages are thus located adjacent to the anode or cathode .

In this embodiment of the invention the metal sheet in the area of the electrochemically active section of the fuel cell can be patterned on either one of the surfaces or on both surfaces, using e.g. chemical etching, mechanical dim^¬ pling or embossing. The protrusions can have any shape, for example being in the form of rods or elongated protrusions. The exact path of the fuel flow can vary, and several fuel paths are possible.

When the metallic element is intended for use as an inter^¬ connect, a reduction of the thermal gradients within the fuel cell is accomplished by ensuring an even fuel gas dis^¬ tribution to the entire cell surface, thereby enabling the endothermic reforming reaction and the exothermic electro^¬ chemical reaction to take place uniformly over the cell surface. The pressure gradients ensure a uniform gas flow over the majority of the cell area.

The metallic element of the invention when used as an in^¬ terconnect is primarily for high temperature application at the fuel gas side of the fuel cell. The oxygen side of the interconnect can have any geometry suitable for the trans^¬ portation of the oxygen. This geometry can e.g. be in the form of straight, parallel channels or any other type known in the art. When the metallic element is used as an interconnect, the presence of protrusions in the interconnect provides it with a much larger surface area and thus improves the re- moval of heat from the anode side to the cathode side. Any type of uniform or non-uniform pattern can be made. The gas flows across the metallic element from the inlet gas pas^¬ sages to the outlet gas passages for the exhaust gases.

The metallic element of the invention can have a thickness of 0.1-5 mm. When the metallic element is a spacer, a thickness of 0.1-1 mm is preferable. When the metallic ele^¬ ment is an interconnect, a thickness of 0.5-5 mm is prefer- able.

The micro channels can extend as straight channels radially from or parallel to each other from the partially open gas passage. Alternatively they can extend radially as curved channels from the partially open gas passage. The micro channels can be discontinuous due to the merging of two mi^¬ cro channels, or they can be continuous. Additionally a mi^¬ cro channel can be subdivided into two micro channels. The micro channels have a width of 0.1-10 mm, and the gas openings can have any shape. The metallic element can be made from a corrosion resistant high temperature alloy. Preferably the metallic element is made from an alumina forming steel when the metallic element is a spacer and from a high temperature alloy when the metallic element is an interconnect.

The interconnect electrically and physically connects the anode of one fuel cell with the cathode of the adjacent fuel cell in a fuel cell stack. The materials that are preferable for the metallic element when it is intended for use as an interconnect must be electrically conducting, oxidation resistant, impermeable to the diffusion of gases and chemically stable towards fuel cell materials.

The Figs. 1-7 illustrating the metallic element of the in- vention are described in more details in the following sec^¬ tions. An overview of the reference numbers used in the Figs, is given below:

Ref. no. Definition

1 Metallic element

2 Metal sheet

3 First planar surface

4 Second planar surface

5 First periphery

6 Second periphery

7 Third periphery

8 Fourth periphery

5a, 6a, 7a, 8a inner border of periphery

5b, 6b, 7b, 8b outer border of periphery

9 inlet gas opening

99 partially open inlet gas opening

10 outlet gas opening

100 partially open outlet gas opening

11 closed gas opening

12 central free space

13 micro channel

14 first open end of micro channel

15 second open end of micro channel

16 first side of micro channel

17 second side of micro channel

18 contact point

W distance from one side to another side L distance between first and second open ends

D etching depth

In the following, a detailed description of the Figs, is given:

Fig. 1 shows a metallic element 1 which is a conventional planar spacer suitable for distribution of gases to both cathode and anode in a fuel cell. The spacer has gas open- ings 9, 10 and 11 along its four peripheries 5, 6, 7 and 8. Gas openings 9 and 10 are open in the plane of the spacer along two opposing peripheries 5 and 7 to allow one of the two gases entering the stack to flow across either the anode or the cathode surface. Gas openings 11 are closed in the plane of the spacer along the other two opposing pe^¬ ripheries 6 and 8 to prevent the flow of the second gas across the same surface as the first gas.

The four peripheries 5, 6, 7 and 8 enclose the central free space 12 which is a free space adjacent to, i.e. either above or below, the electrochemically active area defined by the cathode or the anode of the fuel cell in the stack.

Fig. 2 shows an embodiment of a metallic element 1 of the invention suitable as a spacer. This metallic element 1 consists of a metallic sheet 2 defined by first and second planar surfaces 3 and 4, respectively, on opposite sides of the metallic sheet, and having four peripheries 5, 6, 7 and 8 and at least one gas opening 9, 10 or 11 on first and third opposing peripheries 5 and 7, for transfer of gas to and from the manifold to the fuel cell. The other two op- posing peripheries, that is, the second and fourth periph^¬ eries 6 and 8, are not provided with any gas openings.

The metallic sheet 2 is provided with micro channels 13 ex- tending from partially open inlet gas opening 99 to the central free space 12, where the gas contacts the electro- chemically active section of the fuel cell. After termi^¬ nated reaction the exhaust gases are transferred from the central free space 12 to micro channels 13 associated with the partially open outlet gas openings 100 for removal from the fuel cell stack.

The embodiment shown in Fig. 2 is visualised from the first planar surface 3. The first periphery 5 has three closed gas openings 11 which prevent a second gas from entering the first planar surface 3 of the metallic element 1. Said first periphery 5 also has three partially open inlet gas openings 99 through which a first gas enters the first pla^¬ nar surface 3, from where it is transferred through micro channels 13 as mentioned earlier. Closed gas openings 11 alternate with partially open inlet gas openings 99.

The third periphery 7 on the first planar surface 3 is po^¬ sitioned opposite the first periphery 5, and it is also provided with three closed gas openings 11 and three par^¬ tially open outlet gas openings 100 for exhaust gas re^¬ moval. Closed gas openings 11 alternate with partially open outlet gas openings 100. In this embodiment the micro chan^¬ nels 13 extend radially from the gas openings 99 and 100.

Second and fourth peripheries, 6 and 8 respectively, are without gas openings and have a width much smaller than that of the first and third peripheries 5 and 7. The pe^¬ riphery width is defined as the distance between the outer border 5b, 6b, 7b, 8b (identical to the outermost edge of the metallic element 1) and the inner border 5a, 6a, 7a, 8a (being the edge of the periphery exposed to the electro- chemically active section) . This width can be minimised since no gas openings are required on peripheries 6 and 8, thus maximising the surface area available for electro^¬ chemical reaction.

Fig. 3 shows a section of the metallic element of the in^¬ vention shown in Fig. 2. The metallic element 1 has a first planar surface 3 and a section with a gas opening that is partially open due to the presence of three micro channels 13 etched into its surface. The second planar surface 4 of the metallic element 1 is without micro channels.

Each micro channel 13 has a length L defined as the dis^¬ tance between the first and second open ends 14 and 15 re- spectively. The width W of each micro channel is defined as the distance between its sides 16 and 17. The depth D of each micro channel is defined by the etching depth which is pre-determined . Each micro channel 13 can have the same width throughout its length L, or the micro channel 13 can be diverging, such that its width W at the first open end 14, located at the gas opening 99 or 100, is smaller than its width at the second open end 15 located for instance at the electrochemically active section of the metallic ele^¬ ment .

The width and length of a micro channel does not have to be identical to the width and length of a neighbouring micro channel. This is shown in the Fig. where the dimensions of the central micro channel are not identical to the dimen^¬ sions of the two neighbouring micro channels. In this em^¬ bodiment the micro channels extend radially from the gas opening.

Fig. 4 shows a metallic element of the invention with five partially open inlet gas openings 99 and five closed gas openings 11 on the first periphery 5 and likewise on the third periphery 7. Each partially open inlet and outlet gas opening 99 and 100 has two parallel micro channels 13 ex^¬ tending from the gas opening to the inner edge 8b of the first and third peripheries 5 and 7. The remaining details, including the reference numbers, are identical to those of Fig. 2.

Fig. 5 shows a planar surface 3 of a metallic element 1 of the invention, which is useful as an interconnect. In this embodiment the metallic element 1 of the invention is a me- tallic plate, which has one partially open inlet gas open^¬ ing 99 and two closed gas openings 11 on the first periph^¬ ery 5 and likewise on the third periphery 7. Each partially open inlet and outlet gas opening, 99 and 100 respectively, has several micro channels 13 extending from the gas open- ing 99 or 100 to the inner edge 8a of the first and third peripheries 5 and 7. Some of the micro channels diverge continuously and form micro sub-channels, while others merge from two channels to one. The metallic element 1, when being useful as an intercon^¬ nect, is without a central free space 12, and the metallic element is a continuous plate with discontinuous micro channels, dimples, rods or protrusions in the electrochemi- cally active section, which function as contact points 18 with the elements of the fuel cell stack positioned above the first planar surface. The discontinuous micro channels, dimples, rods or protrusions simultaneously provide mean^¬ dering gas flow distribution paths across the metallic ele^¬ ment .

Fig. 6 shows a planar surface of the metallic element 1, which is useful as an interconnect. In this embodiment the metallic element 1 of the invention is a metallic plate, which has two partially open inlet gas openings 11 and one closed gas opening 99 on the first periphery 5, and like^¬ wise there are two partially open outlet gas openings 11 and one closed gas opening 100 on the third periphery 7. Each of the partially open inlet and outlet gas openings 11 has several curved micro channels 13 extending from the gas openings 11 to the inner edge 8a of the first and third pe^¬ ripheries 5 and 7. Some of the micro channels diverge and form micro sub-channels.

The metallic element 1 being useful as an interconnect is without a central free space 12, and the metallic element is a continuous plate with discontinuous micro channels, dimples, rods or protrusions in the electrochemically ac^¬ tive section, which function as contact points 18 with the elements of the fuel cell stack positioned above the first planar surface. The discontinuous micro channels, dimples, rods or protrusions simultaneously provide meandering gas flow distribution paths across the metallic element.

Claims

1. Metallic element for a high temperature fuel cell stack, each individual fuel cell comprising an anode, a cathode, an electrolyte and an electrochemically active area, the metallic element 1 comprising a metallic sheet 2 having first and second planar surfaces 3 and 4 respec^¬ tively on opposite sides of the metallic sheet 2 and a first, second, third and fourth periphery 5, 6, 7 and 8 re- spectively, the first and third peripheries 5 and 7, and the second and fourth peripheries 6 and 8 respectively forming opposing peripheries, the metallic sheet 1 being provided with inlet and outlet gas openings 99 and 100 ex^¬ tending completely through the metallic sheet from the first planar surface 3 to the second planar surface 4 and positioned at first and third peripheries 5 and 7 only of the metallic sheet 2, at least one inlet gas opening 99 and at least one outlet gas 100 opening being partially open and extending into one or more micro channels 13 located on the first planar surface 3, each micro channel 13 extending continuously or discontinuously from the at least one inlet gas opening 99 or at least one outlet gas opening 100 adja^¬ cent to the electrochemically active area of the fuel cell.

2. Metallic element according to claim 1, wherein micro channels are formed by etching or pressing of the metallic sheet to a predetermined depth in the metallic sheet.

3. Metallic element according to claim 1, wherein the second planar surface is provided with either micro chan^¬ nels or conventional straight channels.

4. Metallic element according to claim 1, wherein the first and third peripheries each comprise at least one gas opening that is closed.

5. Metallic element according to claim 4, wherein the second and fourth peripheries do not comprise any inlet gas or outlet gas openings.

6. Metallic element according to claim 1, wherein the me- tallic sheet comprises a central free space adjacent to and having the same surface area as the electrochemically ac^¬ tive area.

7. Metallic element according to claim 6, wherein the central free space is devoid of metallic material.

8. Metallic element according to claim 1, wherein the me^¬ tallic sheet is provided with a plurality of dimples, rods or protrusions providing flow passages in the metallic sheet adjacent to the electrochemically active area of the fuel cell.

9. Metallic element according to claim 1, wherein the me^¬ tallic sheet is prepared from a corrosion resistant high temperature alloy, preferably from an alumina forming steel when the metallic element is a spacer and from a high tem^¬ perature alloy when the metallic element is an intercon^¬ nect .

10. Metallic element according to claim 1, wherein the me^¬ tallic sheet has a thickness of 0.1-5 mm.

11. Solid oxide fuel cell (SOFC) stack comprising a metal^¬ lic element according to claim 1.

12. Molten carbonate fuel cell (MCFC) comprising a metal- lie element according to claim 1.

13. Solid oxide electrolysis cell (SOEC) comprising a me^¬ tallic element according to claim 1.

14. Metallic element according to claim 1, which element is an interconnect.

15. Metallic element according to claim 1, which element is a spacer.

16. Interconnect according to claim 14 for a high tempera^¬ ture fuel cell stack, each fuel cell comprising an anode, a cathode, an electrolyte and an electrochemically active area, said interconnect being a metallic sheet having first and second planar surfaces on opposite sides of the metal^¬ lic sheet and a first, second, third and fourth periphery, the first and third peripheries, and the second and fourth peripheries respectively forming opposing peripheries, the metallic sheet being provided with inlet and outlet gas openings extending through the metallic sheet from the first planar surface to the second planar surface and posi^¬ tioned at first and third peripheries only of the base sheet, at least one inlet gas opening and at least one out^¬ let gas opening being partially open by extending into one or more micro channels located on the first planar surface, each micro channel extending continuously or discontinu- ously from the at least one inlet gas opening across the metallic sheet adjacent the electrochemically active area of the fuel cell to the at least one outlet gas opening.

17. Spacer according to claim 15 for a high temperature fuel cell stack, each fuel cell comprising an anode, a cathode, an electrolyte and an electrochemically active area, said spacer being a metallic sheet having first and second planar surfaces on opposite sides of the metallic sheet and a first, second, third and fourth periphery, the first and third peripheries, and the second and fourth pe^¬ ripheries respectively forming opposing peripheries, the metallic sheet being provided with inlet and outlet gas openings extending through the metallic sheet from the first planar surface to the second planar surface and posi- tioned at first and third peripheries only of the base sheet, at least one inlet gas opening and at least one out^¬ let gas opening being partially open by extending into one or more micro channels located on the first planar surface, each micro channel extending continuously or discontinu- ously from the at least one inlet gas opening or at least one outlet gas opening to the electrochemically active area of the fuel cell, the metallic sheet comprising a central free space devoid of metallic material adjacent to and hav^¬ ing same surface area as the electrochemically active area.