WO1998025316A1

WO1998025316A1 - Material for fuel element interconnectors

Info

Publication number: WO1998025316A1
Application number: PCT/DE1997/002669
Authority: WO
Inventors: Uwe Diekmann; Willem Quadakkers
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 1996-12-06
Filing date: 1997-11-12
Publication date: 1998-06-11
Also published as: DE19650704C2; DE19650704A1

Abstract

In a batch of solid-oxide fuel elements, the various elements are electrically connected through interconnectors. The solid mechanical linkage required for a good electric contact between elements and interconnectors bring about, in case of significant temperature changes of possibly hundreds of degrees when the fuel elements are activated, mechanical strains due to the fact that the interconnector behaviour with regard to thermal expansion is not sufficiently adapted to the various elements. For the interconnectors intended to link the solid-oxide fuel elements according to the film pattern, there exists of course appropriate materials, but these are too expensive and their workability is far from satisfactory. Besides, they are not suited to interconnectors to which the solid-oxide fuel elements are to be linked according to the substrate pattern. This is due to the fact that the thermal expansion is not alike with both types of fuel element. As an interconnector (18) material electrically connecting the fuel elements (10) in one batch (26), it is suggested that a iron alloy be used which contains 13 to 24 wt.% of chromium and no more than 3 wt.% of manganese and/or no more than 2 wt.% of alumunium and/or no more than 1.5 wt.% of silicon.

Description

MATERIAL FOR FUEL CELL INTERCONNECTORS

The invention relates to a material for interconnectors for the electrical connection of fuel cells of a fuel cell stack.

In fuel cells, the chemical energy stored in the fuel is converted directly into electrical energy and heat. Pure hydrogen, methanol or natural gas are used as fuels, for example, which react in the fuel cell with the oxidant, usually pure or the oxygen contained in air. In addition to electrical power and heat, this reaction also generates water, and carbon dioxide also generates carbon dioxide. Fuel and oxidant are also summarized under the term equipment.

The individual fuel cell has an anode and a cathode, between which the electrolyte is arranged. The fuel is continuously fed to the anode side, the oxidant to the cathode side of the fuel cell, and the reaction products are continuously removed.

The different types of fuel cells are usually classified based on the electrolyte used. A ceramic is used as the electrolyte in the solid oxide fuel cell (SOFC, abbreviation for Solid Oxide Fuel Cell). In contrast to fuel cell types, the electrolyte of the solid oxide fuel cell is solid. The working temperatures of the solid oxide fuel cells range from about 600 to about 1000 ° C.

The materials for the components of the solid oxide fuel cell are predominantly ceramics, the desired electrical and electrochemical properties of which are achieved through the targeted combination and processing of the starting materials. The electrolyte is, for example, a gas-tight ceramic layer made of yttrium-stabilized zirconium dioxide (abbreviated to YSZ), which has a high conductivity for oxygen ions at the operating temperatures between 600 and 1000 ° C. A nickel cermet is generally used for the anode and YSZ, a perovskite based on lanthanum manganite is used for the cathode. The porosity of the two electrode layers must be sufficiently high that, on the one hand, when the fuel cell is in operation, a sufficiently large amount of gas can reach the corresponding electrode / electrolyte interface and, on the other hand, the reaction products can escape unhindered.

Without current flow, a cell voltage of approximately 1 V builds up between the anode and cathode. Since this value is too low for practical use, several individual cells are combined in one module and electrically connected in series. In the flat cell concept, for example, this is done by producing the individual cells in the form of plates and stacking them on top of one another. An electrically conductive plate, the so-called interconnector, is arranged between each two adjacent individual cells and electrically connects the anode of one individual cell to the cathode of the other individual cell. The interconnector is also gas-tight and its two main surfaces have a rib structure, so that open channels are formed to the electrodes. As a result, separate gas spaces are formed between the anode and interconnector on the one hand and between the cathode and interconnector on the other hand, so that the anode can be supplied with fuel and the cathode with air.

Good contact between the interconnector and the electrode is required for a secure electrical connection between the electrode and the interconnector. This is achieved in that, for example, the entire stack of fuel cells and interconnectors is pressed together over a large area, or in that the interconnectors and the electrodes are connected to one another at the desired contact points via suitable contact layers.

To start up the fuel cell stack, it must be heated from room temperature to operating temperature. This change in temperature of a few hundred ° C can be combined with the above-mentioned press or adhesive connection required for electrical contact between the tconnectors and single cells lead to mechanical stresses that are so strong that the break-sensitive ceramic single cells are destroyed. It is therefore extremely important that the different materials are matched to one another in their thermal expansion behavior.

So far, conventional high-temperature nickel-based alloys have been used as

Material used for the manufacture of interconnectors for solid oxide fuel cells. However, satisfactory results could not be achieved with these alloys, since their thermal expansion is much too great in comparison with the ceramic materials used for the individual cells, so that an adaptation was not possible.

It is also known that interconnectors made from oxide dispersion strengthened (abbreviated as ODS) chromium-based alloys, for example with the composition 5% by weight Fe, 1% by weight 2O3, the rest Cr (abbreviated

or from ceramics based on lanthanum chromite (LaCrθ3). Because of their thermal expansion behavior, such interconnectors are well suited for stacks of solid oxide fuel cells which are constructed according to the so-called film concept.

This film concept states that the mechanical stability of the individual cell is mainly due to the electrolyte. The structure of such a single cell looks, for example, in such a way that the electrolyte is a 100-300 μm thick, flat, self-supporting film made of the above-mentioned YSZ, on one side of which the anode from the above-mentioned cermet and on the other side of which Cathode from the above-mentioned perovskite can be applied in 50-100 μm thick layers. The dimensions of such individual cells are limited by their mechanical stability and their manageability in the manufacturing and further processing process. Single cells of 100 x 100 mmA are common. The thermal expansion behavior of this single cell is primarily determined by the electrolyte layer, which has a very low thermal expansion. In addition to the film concept, the so-called substrate concept has recently also been followed, which states that it is not the electrolyte that provides mechanical stability, but a substrate layer. This can be the anode, for example. This is advantageous since the ohmic losses of the anode are lower than that of the cathode and much lower than that of the electrolyte. The

Anode is for example 2000 μm thick and with this thickness even with large ones

Areas of, for example, 250 x 250 mm ^{2 are} still sufficiently stable. For example, an approximately 20 μm thick electrolyte layer and an approximately 50 μm thick cathode layer are applied to this anode substrate.

Other structures are also possible, for example a self-supporting substrate layer, which does not have to have any influence with regard to the electrochemical processes in the fuel cell, on which the actual fuel cell layers, i.e. anode, cathode and electrolyte, are built up. These can then be applied as thin as desired, since they no longer have to contribute to the mechanical stability of the individual cell. However, the substrate layer must be such that the electrode applied directly to it can be supplied with sufficient resources.

Since the electrolyte in the substrate concept is much thinner than in the film concept, the operating temperature can be reduced below 700 ° C. Although the lowering of the operating temperature is associated with a reduction in the specific electrical conductivity of the electrolyte, this effect is compensated for by the shorter distance that the oxygen ions have to travel through the electrolyte on their way to the anode.

The thermal expansion behavior of the individual cells in accordance with the substrate concept is determined primarily by the self-supporting substrate, in the first example mentioned by the anode. However, since this generally has a higher thermal expansion than the electrolyte, which has a very low thermal expansion, the known interconnector materials are not suitable here. A major disadvantage of the known ODS-Cr alloys is that during operation of the fuel cell stack, cover layers made of chromium oxide form on the surface of the interconnector and evaporate into the respective gas space. This evaporated chromium oxide contaminates the individual cells, especially the cathodes, which is a serious aging problem.

In addition, ODS-Cr alloys are manufactured in a powder-metallurgical and thus complex process and are therefore very expensive. Their low fracture toughness and the associated poor processing properties are also very disadvantageous.

The well-known ceramics based on lanthanum chromite are high

Raw material and manufacturing costs as well as the specific electrical conductivity that is present at the high operating temperatures but not satisfactory in comparison to the metallic materials are significant disadvantages.

It is therefore an object of the invention to provide a material of the type mentioned at the outset which overcomes the disadvantages associated with the known materials.

This object is achieved by an iron-based alloy which has 13 to 24% by weight of chromium and at most 3% by weight of manganese and / or at most 2% by weight of aluminum and / or at most 1.5% by weight of silicon .

The thermal expansion behavior of the material according to the invention fits well with the individual cells according to the substrate concept, it is simple and inexpensive to manufacture, it has very good processing properties and high electrical conductivity.

The material according to the invention is stable at the high operating temperatures. It also withstands the conditions on the cathode side, for example, where there is a very oxidizing atmosphere, and those on the anode side, where hot water vapor is present, for example. This is necessary because the interconnector delimits the gas space of the anode on one side and the gas space of the cathode on the other side. As a metallic material, it also has very good specific electrical conductivity.

In addition, each of the alloy elements Al, Si and Mn, alone or together with one of the two other alloy elements or with the two other alloy elements, prevents the formation of pure chromium oxide layers. Instead, cover layers are formed from mixed oxides or oxide mixtures, which have very good oxidation resistance and allow extremely reduced evaporation of contaminating chromium oxides.

The fuel cells therefore age much more slowly.

An advantageous example of this material has a content of 0 to 0, 12 wt .-% carbon, 12 to 14 wt .-% chromium, 0 to 1 wt .-% manganese, 0.7 to 1.2 wt .-% Aluminum and 0.7 to 1.4 wt .-% silicon.

Another advantageous example of this material has a content of 0 to 0, 12 wt .-% carbon, 17 to 19 wt .-% chromium, 0 to 1 wt .-% manganese, 0.7 to 1.2 wt .-% % Aluminum and 0.7 to 1.4% by weight silicon.

Another advantageous example of this material has a content of 0 to 0, 12 wt .-% carbon, 23 to 26 wt .-% chromium, 0 to 1 wt .-% manganese, 1.2 to 1.7 wt .-% % Aluminum and 0.7 to 1.4% by weight silicon.

Further advantageous compositions and advantageous applications and developments are described in the subclaims.

The material according to the invention is particularly suitable for operating temperatures of at most 900 ° C.

An interconnect can advantageously be produced from the material according to the invention, which electrically and mechanically connects the individual cells in a fuel cell stack. Due to its thermal, electrical and electrical Such an interconnector is particularly suitable for high-temperature fuel cells due to its chemical properties.

Preferred areas of application of the invention are described below with reference to the accompanying drawings.

FIG. 1 is a schematic illustration of a solid oxide fuel cell;

FIG. 2 is an enlarged cross section through a solid oxide fuel cell according to the film concept;

FIG. 3 is an enlarged cross section through a solid oxide fuel cell according to the substrate concept; FIG. 4 is a perspective view of an interconnector;

FIG. 5 is a partially broken perspective view of a fuel cell assembly that includes a stack of solid oxide fuel cells according to the substrate concept and interconnectors according to FIG. 4; FIG. 6 is a along line VI-VI in FIG. 5 sectional detailed view of the fuel cell stack; and

FIG. 7 is a graph plotting the relative thermal expansion of various materials used for solid oxide fuel cells and interconnectors versus temperature.

Operation and structure of high-temperature fuel cells are described below using the example of a solid oxide fuel cell operated with hydrogen and air.

According to FIG. 1 to 3, a solid oxide fuel cell 10 has an anode 12, an electrolyte 14 and a cathode 16. The electrolyte 14 is a gas-tight ceramic layer made of YSZ, which consists of Zrθ with an addition of 8 mol% Y2O3. The anode 12 is made of a Ni-YSZ cermet, which is made from the starting materials YSZ, which consists of Zrθ with an addition of 8 mol% Y2O3, and NiO. The cathode 16 is made of a perovskite based on lanthanum manganite with the composition Lao Q ^ Q 3oMnθ3- The two electrodes layers are gas-permeable, so that when the fuel cell 10 is operating, the hydrogen reaches the anode / electrolyte interface and the atmospheric oxygen reaches the cathode / electrolyte interface in sufficient quantities and, on the other hand, the reaction product water can escape unhindered.

The according to FIG. 1 at the cathode / electrolyte interface layer from the continuously supplied atmospheric oxygen, O ² "ions migrate through the electrolyte 14 to the anode / electrolyte interface layer. There the hydrogen is oxidized and reacts with the O ^2_ ions to form water, in addition to the heat of reaction electrons are also released. These flow back via a consumer connected between anode 12 and cathode 16 to cathode 16, where they form new O ^{2 "} ions. The water formed at the anode 12 is present as steam because of the high temperatures and, like the air which is reduced in its oxygen content, is continuously removed on the cathode side.

In FIG. 2 shows the structure of a planar solid oxide fuel cell 10 according to the film concept, in which the mechanical stability of the individual cell 10 is provided by the electrolyte 14. This is a 150 μm thick, flat film from the YSZ mentioned above. The anode 12 and the cathode 16 are each 50 μm thick layers made of the above-mentioned materials, which are applied to the electrolyte film on both sides.

FIG. 3 shows the structure of a solid oxide fuel cell 10 according to the

Substrate concept in which the same materials as in the in FIG. 2 shown solid oxide fuel cell 10 are used according to the film concept. In this single cell 10, however, the supporting substrate is a 2000 μm thick anode 12. The electrolyte layer with a thickness of 20 μm and the cathode layer with a thickness of 50 μm are applied to this anode substrate.

FIG. 4 shows a plate-shaped interconnector 18, which is made of an iron alloy according to the invention with a content of 0 to 0, 12% by weight carbon, 17 to 19% by weight chromium, 0 to 1% by weight manganese, 0.7 to 1 , 2 wt .-% aluminum and 0.7 to 1.4 wt .-% silicon. The layout of the interconnector 18 is essentially the same as that of the individual cells 10, in the present preferred embodiment it is square, but it can also have a different shape. The two square main surfaces 20, 22 of the interconnector 18 are ribbed in such a way that a plurality of parallel, groove-shaped channels 24 extend continuously from one edge of the

Interconnector 18 to the opposite range. The channels 24 'run in the manner shown in FIG. 4 visible upper main surface 20 perpendicular to the channels 24 ″ in the opposite lower main surface 22.

The in FIG. 5 shows a fuel cell stack 26 and four gas boxes attached to it. The fuel cell

Stack 26 includes ten solid oxide fuel cells 10, each of which is shown in FIG. 3 substrate concept shown is constructed. In each individual cell 10, the anode 12 is at the top, the cathode 16 at the bottom. Two adjacent individual cells 10 are connected by an interconnector 18 according to FIG. 4 on the one hand spatially separated from one another, on the other hand mechanically and electrically connected by this, as will be described in more detail below. On the anode 12 of the uppermost individual cell 10 and below the cathode 16 of the lowest individual cell 10 there is also an interconnector 18 ', 18 ". The uppermost interconnector 18' differs from the other nine interconnectors 18 between two individual cells 10 in that only the lower main surface 22 abutting the anode 12 has the channels 24 ″, whereas the upper, free main surface 20 ′ is flat. Correspondingly, the lowest interconnector 18 "differs from the remaining nine interconnectors 18 between two individual cells 10 in that only the upper main surface 20 adjacent to the cathode 16 has the channels 24 ', whereas the lower, free one

Main surface 22 "is flat. A current collector tab 36 is welded to each of these free main surfaces 20 ', 22', via which the electrical current generated in the fuel cell stack 26 is dissipated.

Gas boxes 28, 30, 32, 34 are attached to each of the four side surfaces of the stack 26 in an airtight manner, via which the operating means are respectively supplied or removed. The in FIG. 5 front gas box 28 is used to supply air that is tere gas box 30 of the removal of air reduced in oxygen content. The in FIG. 5 left gas box 32 is used to supply hydrogen, the right gas box 34 is used to remove the water and the hydrogen that has not reacted. The joints between the gas boxes and the stack 26 are sealed with glass solder.

FIG. 6 is a section through the fuel cell stack 26 along the line VI-VI in FIG. 5 and shows in an enlarged detail how the anode 12 and cathode 16 of an individual cell 10 are contacted with the corresponding interconnector 18. The in FIG. 6 left side surface of the stack 26 has, as also in FIG. 5, to the hydrogen supply box 32.

In FIG. 6 one of the channels 24 "running from left to right in the lower main surface 22 of the upper interconnector 18 is shown in longitudinal section. Hydrogen flows through this channel 24" from the left out of the hydrogen supply box 32 to the anode 12. Furthermore, FIG. 6 shows two of the channels 24 ′ running from the front to the rear in the upper main surface 20 of the lower interconnector 18 in cross section. Air flows from the air supply box 28 to the cathode 16 through these channels 24 ′ from the front.

The electrical contacting of the electrodes 14, 18 with the interconnector 18 takes place on the anode side with the aid of a nickel network 38, which is achieved by spot welding on the webs 40 ″ delimiting the channels 24 ″ on the lower one

Main surface 22 of the interconnector 18 is fastened and pressed onto the anode 12 by the dead weight of the interconnectors 18 and individual cells 10 lying above it. On the cathode side, a contact layer 42 made of a ceramic based on lanthanum cobaltite is provided between the webs 40 on the upper main surface 20 of the interconnector 18 and cathode 16.

According to FIG. 6, the cathode layer does not reach all the way to the edge of anode 12 and electrolyte 14. Rather, the underside of the electrolyte layer is exposed all round. This two-layer edge region 44 of the individual cell 10, which runs around the entire circumference of the individual cell 10, is provided with a sealing mass 46 enclosed, which consists of alkali silicate glass with additions of MgO and YSZ and adheres poorly to the cathode material used. This seal prevents, as shown in FIG. 6, it can be seen that the hydrogen present in the hydrogen feed box 32 and in the channel 24 "above the anode 12 mixes with the oxygen in the channels 24 'under the cathode 16. The seal also adheres to the outside Edge regions of the webs 40 in the upper and lower main surfaces 20, 22 of the interconnectors 18, so that interconnectors 18 and individual cells 10 are firmly connected to one another.

Since the electrical contact described also has a fixed mechanical

Connection between interconnectors 18 and single cells 10 entails, the different materials in their thermal expansion behavior must be coordinated to the extent that there is no destruction of the brittle individual cells even with large temperature changes, which occur, for example, when the fuel cell unit is switched on and off 10 is coming.

In the in FIG. The diagram shown in FIG. 7 plots the relative thermal expansion ΔL / LQ as a function of the temperature, which was measured for different materials. The difference between two curves at a given temperature is a direct measure of the mechanical stress that would occur between two corresponding components if this temperature were reached if they had been firmly connected at the initial temperature of 20 ° C.

Curve 1 belongs to an interconnector 18, the composition and structure of which are described above in connection with FIG. 4 is described. The curve 2 belongs to one in connection with FIG. 3 described solid oxide fuel cell 10 according to the substrate concept. Curve 3 belongs to an interconnector made of the above-mentioned known ODS-Cr alloy Cr5FelY ₂ θ3. The curve

4 belongs to one in connection with FIG. 2 described solid oxide fuel cell 10 according to the film concept. It can be clearly seen that curves 1 and 2 on the one hand and curves 3 and 4 on the other hand fit well together. However, the curves 2 and 3 are already so far apart from approximately 200 ° C. that the resulting mechanical stress would destroy the fuel cell 10.

Claims

PATENT CLAIMS

1. Material for interconnectors (18) for the electrical connection of fuel cells (10) of a fuel cell stack (26), characterized by an iron-based alloy containing 13 to 24% by weight of chromium and a maximum of 3% by weight of manganese and / or at most 2% by weight of aluminum and / or at most 1.5% by weight of silicon.

2. Material according to claim 1, characterized in that the manganese content is at least 0.5 wt .-%.

3. Material according to claim 1 or 2, characterized in that the aluminum content is at least 0.5 wt .-%.

4. Material according to any one of the preceding claims, characterized in that the silicon content is at least 0.5% by weight.

5. Material according to claim 1, characterized in that it contains 0 to 0, 12 wt .-% carbon, 12 to 14 wt .-% chromium, 0 to 1 wt .-% manganese, 0.7 to 1, Has 2 wt .-% aluminum and 0.7 to 1.4 wt .-% silicon.

6. Material according to claim 1, characterized in that it contains 0 to 0, 12 wt .-% carbon, 17 to 19 wt .-% chromium, 0 to 1 wt .-% manganese, 0.7 to 1, Has 2 wt .-% aluminum and 0.7 to 1.4 wt .-% silicon.

7. Material according to claim 1, characterized in that it contains 0 to 0, 12 wt .-% carbon, 23 to 26 wt .-% chromium, 0 to 1 wt .-% manganese, 1.2 to 1, 7 wt .-% aluminum and 0.7 to 1.4 wt .-% silicon.

8. interconnector for electrical connection of fuel cells (10) of a fuel cell stack (26), characterized in that it is made of a material according to one of the preceding claims.

9. Use of a material according to one of claims 1 to 7 for the manufacture of an interconnector (18) for the electrical connection of fuel cells (10) of a fuel cell stack (26).

10. Use according to claim 9, characterized in that the fuel cells (10) are substrate-based high-temperature fuel cells.

11. Use according to claim 10, characterized in that the material is exposed to operating temperatures of at most 900 ° C.