WO2008064938A1

WO2008064938A1 - High-temperature fuel cell with ferritic component

Info

Publication number: WO2008064938A1
Application number: PCT/EP2007/060020
Authority: WO
Inventors: Marco Brandner; Dr. Leszek Niewolak; Jan Froitzheim; Willem J. Quadakkers; Frank Tietz; Thomas Dr. HÖFLER
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2006-11-27
Filing date: 2007-09-21
Publication date: 2008-06-05
Also published as: DE102006056251A1; DE102006056251B4

Abstract

The invention relates to a high-temperature fuel cell in addition to a method for operation. The object of the invention is to provide a powerful high-temperature fuel cell with long-term stability using ferritic material. In order to minimize any interaction between ferritic components and nickel components of an SOFC, a barrier layer is provided between the problematic components, i.e. for example between the interconnector and the nickel net or between the metal substrate and the anode. A barrier layer in the sense of the invention is present if a layer is provided which reduces or suppresses the interdiffusion of critical elements, primarily iron, chromium and nickel. If there is no such layer, there is then increased interdiffusion.

Description

High temperature fuel cell with ferritic component

The invention relates to a Hochtemperαturbrennstoffzelle together with a method for operation.

A fuel cell is an electrochemical energy converter, consisting of anode, electrolyte and cathode, which can convert the chemical energy of a fuel without combustion process into electrical energy. In the case of the high temperature fuel cell (SOFC) that is

Electrolyte material a gas-tight oxygen ion conductor. A sufficient ionic conductivity is given depending on the material and the thickness of the electrolyte between 500 ⁰ C and 1 000 ⁰ C. According to the current state of the art, yttria-stabilized zirconia (YSZ) is mostly used. The cathode of a SOFC regularly consists of one

Ceramic with perovskite crystal structure. For the anode usually a cermet (ceramic / metal composite material) of YSZ and nickel is used.

A planar design of the cells with a thin electrolyte layer of a few micrometers is advantageous in order to realize high power densities at the same time low operating temperature, preferably below 800 ⁰ C. Mostly, the mechanical stability of such cells is ensured by a cermet anode substrate having a thickness of 0.3 to 2 mm. In order to achieve the required overall performance for a system, fuel cells are electrically connected in series with the aid of so-called interconnectors, for example known from DE 10200501 4077A1 or DE 1020050051 1 6Al, and built up into stacks.

An operating temperature below 800 ^° C allows the use of ferritic steels for components of the SOFC. Interconnectors made of ferritic chromium steels have already been extensively studied and have proven to be suitable. Furthermore, attempts are being made to use porous ferritic supporting structures instead of the established cermet anode substrates in order to ensure stability with respect to mechanical or thermal to improve induced voltages. As a rule, this support structure is used on the anode side. The functional cell layers can be applied by thermal coating methods, physical coating methods or sintering technology.

When ferritic steels are used in SOFCs, they are in direct, cohesive contact with components containing nickel, according to current concepts. It is known to realize the electrical contact between ceramic cells and interconnectors via nickel networks. If porous, ferritic metal substrates are used on the anode side, these are usually in contact with a nickel-containing anode.

Adverse interdiffusion occurs at interfaces between steel and nickel, with the elements iron, chromium and nickel in particular being involved. However, trace elements of the alloys used, such as, for example, manganese in CroFer22APU (Thyssen Krupp VDM) or impurities in the reaction, may also be involved.

The interactions take place during both manufacturing processes and during operation. The contact area between an interconnector and a nickel network is subject to current concepts a high-temperature cycle during the addition of a stack and the continuous load during operation. For the contact area between a metallic substrate and an anode is still a temperature load in the context of cell production added. If the functional layers anode, electrolyte and cathode are applied by thermal spraying, this production step is of short duration at high temperature. If the functional layers are applied by physical coating methods or by sintering, the reactions take place over several hours. The interdiffusion of the elements iron, chromium and nickel has negative consequences for the ferritic component (interconnector or metallic substrate) as well as for the nickel component (nickel mesh or nickel-containing anode).

The introduction of nickel into the ferritic steel can lead to an increase in the oxidation rate and thus to a shortening of the oxidation-related lifetime. In addition, from a nickel content of about 1 0 wt.% An austenitization of the steel is to be expected. The associated increase in the coefficient of thermal expansion leads to a mismatch and tensions between the ceramic cell layers and the steel component. Loss of performance or complete cell failure due to cracking and consequent loss of electrical contact may result.

The diffusion of iron and chromium into nickel during the operation of an SOFC leads to the formation of iron oxides and chromium oxides on free nickel surfaces or along grain boundaries. Since the oxidation is accompanied by an increase in volume, significant stresses can be induced in a structure that leads to a destruction of integrity. The formation of oxides, especially chromium oxide, on the nickel surface also leads to a passivation of the catalytically active material.

For a composite of a porous, ferritic substrate and a nickel-containing anode, the interdiffusion is considered to be particularly critical. On the one hand, the extent of the interactions is greater than in the contact area of a nickel mesh with the interconnector, because cell production already leads to a reaction. Second, the passivation of the nickel surface leads to a reduction of the catalytically active three-phase interface of an anode, which leads to high power losses during cell operation. It is an object of the invention to provide an efficient and long-term stable high-temperature fuel cell using ferritic steel.

In order to minimize interaction between ferritic components and nickel components of an SOFC, a barrier layer is provided between the problematic components, for example between interconnector and nickel mesh or between metal substrate and anode. A barrier layer according to the invention is present when a layer is present which reduces or eliminates the interdiffusion of critical elements, especially iron, chromium and nickel. So if there is no such layer, then there is an increased interdiffusion.

The barrier layer can be made of Ce ₁ . _x . _y (Y, Ln) _x (Ti, Nb) _y O ₂ , (Ca, Sr, Ba) i. _{3x / 2} (Y, Ln) _x TiO ₃ , Cr ₂ O ₃ , (Cr, Mn) ₃ O ₄ , FeO, Fe ₃ O ₄ or Ln _1. , (Sr, Ca) _x (Mg, Al, Cr) O ₃ (where: Ln = lanthanides) exist. Typical ferritic components or steels are CroFer22APU (Thyssen Krupp VDM), IT-I 1 and IT-I 4 (Plansee) or ZMG232 (Hitachi Metals).

In an advantageous embodiment, the barrier layer is chosen so that it makes only a small contribution to the electrical resistance of the cell. The barrier layer thus consists of a material with high electronic conductivity. The area-specific, electronic conductivity of the barrier layer is therefore preferably above 200 S / cm ² .

In an advantageous embodiment, the barrier layer is designed so that at least on one side, the surface of the metal substrate is protected by a thin (<1 0 microns), dense film, the pore network of the metal substrate is not closed. Alternatively, the barrier layer covers the pores of the metal substrate when the Bαrriereschicht itself is open-pore, so that the passage of gas to the anode is not hindered.

A thin coating of the particle surfaces is especially preferable in order to ensure a small contribution to the resistance of the cell.

In particular, CeO _{2 has} proven to be suitable as the material for the barrier layer, because it effectively prevents the interdiffusion of iron, chromium and nickel, and has a higher electronic conductivity than other materials tested. Furthermore, it is thermodynamically stable throughout the temperature and oxygen partial pressure range during production and operation of the SOFC.

In one embodiment of the invention, the material of the barrier layer dopants to increase the electronic conductivity and / or adjustment of the thermal expansion coefficient of the steel component. Thus, an increase in the electronic conductivity is achieved with a doping of CeO ₂ with niobium, yttrium or samarium. A doping of CeO ₂ with gadolinium reduces the swelling behavior known under anode-side operating conditions of an SOFC, which could lead to stresses in the cell network.

Generally, Ce is ₁ . _x . _y (Y, Ln) _x (Ti, Nb) _y O ₂ with 0≤x≤ 0.5 0 0.2 ≤y≤ particularly well suited as a barrier layer material, since this material has a particularly good conductivity is achieved on a regular basis.

If the barrier layer consists of Ln ,. _x (Sr, Ca) _x (Mg, Al, Cr) O ₃ with 0, 1 ≤x≤0.5 (where: Ln = lanthanides) or from (Ca, Sr, Ba) i. _{3x / 2} (Y, Ln) _x TiO ₃ with 0 ≤x _y ≤ 0, l or 0 ≤x _Ln ≤ 0.67 (where: Ln = lanthanides), a particularly good barrier effect is achieved. By doping can be the increase electrical conductivity and thus provide a suitable material.

If the barrier layer consists of Cr ₂ O ₃ and / or (Cr, Mn) ₃ O ₄ or of FeO and / or Fe ₃ O ₄ , the material costs are low. If very thin layers are provided, the material costs, however, play a subordinate role. In particular, more powerful materials such as CeO ₂ (doped or undoped) are to be preferred.

embodiments

In a first example, a CeO ₂ is Diffusionsba rriere between a ferritic metal substrate and a sintered Ni / YSZ anode prepared as follows.

1 . ) Preparation of a pulvermeta llurgischen ferritic metal substrate 2) pre-sintering of the metal substrate at 1 000 ⁰ C, 3 h in argon

Atmosphere 3.) CeO ₂ coating by means of PVD, magnetron sputtering (alternatively:

CVD, SoI gel spincoating or infiltration, sintering, thermal

syringes)

4.) Coating with a NiO / YSZ or Ni / YSZ screen printing layer 5.) Sintering of the composite at 1 250 ⁰ C, 3 h in argon atmosphere 6.) Application of a thermally sprayed electrolyte (alternatively: PVD, Sol

Gel, sintering) 7.) Application of a screen-printed cathode

The result was a fuel cell with a 1 mm thick metal substrate 1 of ferritic Cr steel, a 5 micron thick CeO ₂ - barrier layer 2, a 20 micron thick Ni / YSZ existing anode 3, a 60 micron thick, consisting of YSZ electrolyte. 4 and a 60 μm thick (LaSr) (CoFe) O ₃ existing cathode 5. The CeO ₂ diffusion barrier layer between Metαllsubstrαt and Ni / YSZ anode SOFC ^' s could be produced, which reached in the e zelzelltest power densities of 430 mW / cm ² at 800 ⁰ C and 0, 7 V. Figure 2 shows the measured current, Spαnnungskennlinien at the start and after 1 h 65 continuous operation at 800 ⁰ C and a current density of 0, 3 A / cm ^2. An interdiffusion of the elements iron, chromium and nickel could not be determined in a follow-up examination by scanning electron microscopy (RE M) and X-ray microanalysis (EDX).

By using the CeO ₂ Diffusionsba rriereschicht was at a

Current density of 0, 3 A / cm ² at 800 ⁰ C operation over 1 65 h possible without there was a significant deterioration in cell performance, as shown in Figure 3 illustrates.

A waiver of the diffusion barrier layer resulted in the context of a comparative experiment with otherwise the same structure to a complete cell failure, as it came to the formation of cracks in the electrolyte by the volume increase during the formation of iron oxides and chromium oxides in the anode.

In another embodiment, a CeO ₂ diffusion barrier is formed between a ferritic metal substrate and a thermally sprayed Ni / YSZ anode as follows:

1 . ) Preparation of a pulvermeta llurgischen ferritic metal substrate 2) sintering of the metallic substrate, for example, at 1250 ⁰ C 3) CeO ₂ coating using PVD, magnetron sputtering (alternatively:

CVD, sol gel, sintering, thermal spraying) 4.) Thermal spraying of a NiO / YSZ or Ni / YSZ anode

5.) Thermal spraying of an electrolyte 6.) Thermal spraying of a cathode In a further embodiment, a CeO ₂ diffusion barrier between a ferritic interconnector and a nickel contact element to the ceramic cell is produced as follows:

1.) Production of a SOFC

2.) Production of a ferritic interconnector

3.) CeO ₂ coating of the interconnector by means of PVD,

Magnetron sputtering (alternatively: CVD, SoIGeI, sintering, thermal spraying) 4.) Contact between the SOFC and the interconnector with a nickel contact element (mesh, fabric, knitted fabric, powder)

The features of a high-temperature fuel cell (SOFC) mentioned in the introduction to the description can in principle also be features of the present invention.

Claims

claims

1 . High-temperature fuel cell with a ferritic component (1) and a nickel component (3) and an intermediate barrier layer (2).

2. High-temperature fuel cell according to claim 1, wherein the barrier layer consists of CeO ₂ .

3. High-temperature fuel cell according to claim 1 or 2, wherein the barrier layer is doped with yttrium, gadolinium niobium or samarium.

4. High-temperature fuel cell according to claim 1, 2 or 3, wherein the barrier layer as a thin, preferably dense film on at least one side covers the ferritic component.

5. High-temperature fuel cell according to one of the preceding claims, wherein the barrier layer is not thicker than 1 0 microns, preferably thinner than 2 microns.

6. High-temperature fuel cell according to one of the preceding claims, wherein the ferritic component is a porous metallic substrate (1) and the nickel component is an anode (3).

7. High temperature fuel cell according to one of the preceding claims, wherein the ferritic component is an interconnector and the nickel component is a nickel network.

8. High-temperature fuel cell according to one of the preceding claims, wherein the barrier layer of (Ca, Sr, Ba) i. _{3x / 2} (Y, Ln) _x TiO ₃ with 0 ≤ x _y ≤ 0, l or 0 ≤x _Ln ≤ 0.67 (where: Ln = lanthanides).

9. Hochtemperαturbrennstoffzelle according to any one of the preceding claims, wherein the barrier layer of Cr ₂ O ₃ and / or (Cr, Mn) ₃ O ₄ consists.

1 0. High-temperature fuel cell according to one of the preceding claims, wherein the barrier layer of FeO and / or Fe ₃ O ₄ consists.

1 1. High temperature fuel cell according to any one of the preceding claims, wherein the barrier layer of Ce. ₁ _x . _y (Y, Ln) _x (Ti, Nb) _y O ₂ with 0 ≤ x ≤ 0.5 and 0 ≤y≤ 0, 2 (where Ln = Lantha nide).

1 2. The high-temperature fuel cell according to claim 1, wherein the barrier layer of Ln I -x (Sr, Ca) x (Mg, Al, Cr) O3 with 0, 1 ≤x≤ 0.5

(where Ln = lanthanides).

1 3. A method for operating a high-temperature fuel cell according to any one of the preceding claims, characterized in that the high-temperature fuel cell is not heated during operation to temperatures of more than 800 ⁰ C a.