WO2015009232A1

WO2015009232A1 - A fuel cell and a support layer therefore

Info

Publication number: WO2015009232A1
Application number: PCT/SE2014/050891
Authority: WO
Inventors: Syed Asif Ansar
Original assignee: Saan Energi Ab
Priority date: 2013-07-16
Filing date: 2014-07-14
Publication date: 2015-01-22
Also published as: EP3022789A1; EP3022789A4

Abstract

The present invention describes a solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC) layered structure, said structure comprising a fuel electrode layer; an oxygen electrode layer; an electrolyte layer positioned in between the fuel electrode layer and the oxygen electrode layer; a support layer being attached to the fuel electrode layer and/or the oxygen electrode layer, said support layer being composed of a porous 3D backbone structure and a coated material covering the 3D backbone structure, wherein the 3D backbone structure of the support layer comprises a metal or metal alloy or a mixture of several metals and/or metal alloys and at least one toughened ceramic, and wherein the 3D backbone structure of the support layer is free from any metal exhibiting full reduction-oxidation (redox) transformation at cell operating temperature and conditions and free from any electro-catalytic ceramic or metal. The coated material on the 3D structure guarantees high temperature electrical conductivity sufficient for or superior to the needs for appropriate cell operation.

Description

A FUEL CELL AND A SUPPORT LAYER THEREFORE

Field of the invention

The present invention relates to a layered structure for a solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC).

Technical Background

There are many solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) known today. For instance in US2006/0127746 there is disclosed a fuel cell supported on the electrode side, comprising an anode, electrolyte and cathode, the electrode support comprising a porous part made of an alloy with iron and chromium, said electrode support being a cathode support and said cathode, electrolyte and anode are successively applied thereon and the combination obtained is sintered.

Furthermore, in EP1472755 there is disclosed a fuel cell stack comprising a plurality of tubular fuel cells, each fuel cell comprising an inner electrode layer, an outer electrode layer, and an electrolyte layer sandwiched between the inner and outer electrode layers; and a continuous solid phase porous matrix in which the fuel cells are embedded, wherein a first reactant is flowable through the matrix and to the outer electrode layer of at least one of the fuel cells and a second reactant is flowable through the inside of at least one of the fuel cells and to the inner electrode thereof. The fuel cell stack according to EP1472755 may be of a solid-oxide type and the matrix composition may include an electronic or mixed (ionic and electronic) conductive ceramic, metal or cermet material.

Furthermore, in Matus et al, "Metal-suported solid oxide fuel ceil membranes for rapid thermal cycling" in Solid State Ionics 176 (2005), pp 443-449, another SOFC with a supporting layer is seen. To increase the cells mechanical stability, the authors focuses upon decreasing the coefficient of thermal expansion (CTE) in the support layer to match it better with the other layers of the cell structure. The supporting layer comprising a cermet of a FeCr alloy in which a ceramic has been mixed in. In this case the supporting layer has been applied to the cells anode side.

Another SOFC may be seen in US6051330. This patent concerns a bipolar plate for interconnection between cells. The bipolar plate mentioned is a dense body made up from a super alloy and a cermet of partially stabilized zirconium (PSZ). The super alloy used in the plate is preferably said to be "!nconnel" (NiCrFe). One aim of the present invention is to provide a solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC) layered structure having a porous support layer with improved mechanical properties, such as mechanical strength, fracture resistance / fracture toughness, toughness and durability in relation to thermal transients and cycle, faster start up time, etc and at the same time exhibit high electronic conductivity. Summary of the invention

The stated purpose above is achieved by a solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC) layered structure, said structure comprising:

- a fuel electrode layer;

- an oxygen electrode layer;

- an electrolyte layer positioned in between the fuel electrode layer and the oxygen electrode layer;

- a support layer being attached to the fuel electrode layer and/or the oxygen electrode layer, said support layer being composed of a porous 3D backbone structure and a coated material covering the 3D backbone structure;

wherein the 3D backbone structure of the support layer comprises:

- a metal or metal alloy or a mixture of several metals and/or metal

alloys; and

- at least one toughened ceramic;

and wherein the 3D backbone structure of the support layer is free from:

- any metal exhibiting full reduction-oxidation (redox) transformation at cell operating temperature and conditions; and - any electro-catalytic ceramic or metal,

and wherein the material that is coating the 3D structure is a ceramic and /or metal which exhibit high electronic conductivity at operating temperatures and conditions.

Most Solid Oxide Fuel Cells (SOFC) are made of ceramic materials and commonly supported on 8 mole% yttria stabilized zirconia (8YSZ) electrolyte, referred to as electrolyte supported cells, or 8YSZ + NiO anode, called anode supported cell. Both these cells are referred hereafter as ceramic supported SOFC (CS-SOFC). The support materials and resulting cell structure is fragile and lacks robustness. Consequently the reliability of cells, especially under transient or cycling conditions, is inferior to industrial needs.

In order to enhance the robustness of cells, metal-supported solid oxide fuel cells (MS-SOFC) have been developed. Although MS-SOFC demonstrated clear improvement in reliability under cycling condition compared to CS-SOFC, the life time and power density of MS-SOFC remained mediocre due to high degradation of metal support and inter- diffusion between species of anode and metal substrate. Moreover, excessive oxidation and limited shrinkage of metal support does not enable apply air based sintering methods to produce functional components of the cell.

The present invention is directed to a fuel cell being supported on a porous support that is made of toughened ceramic-metal composite. A toughened ceramic-metal composite differs from hardened ceramic-metal composite. In this case the toughened ceramic-metal composite is considered more though and also able to bend and flex more, whereas the hardened ceramic-metal composite is considered more brittle and rigid.

One advantage using toughened ceramic-metal is that the cells mechanical strength, fracture resistance / fracture toughness, toughness and durability in relation to thermal transients and cycle is greatly enhanced. However any support just containing only two of these components may not have sufficient electrical conductivity. Hence in the invention the toughened ceramic-metal support is further coated internally with a third component which guarantees conductivity suffieceint or superior to the requirements for these cells. Cells using Inconel (NiCrFe), either as a separate support structure or in a bipolar plate, does not allow for sufficient electronic conductivity, as an oxide acting as an isolator will form at high temperature, leading to a high electronic resistance.

To adjust the coefficient of thermal expansion (CTE) for the Inconel one may use a ceramic material as according to prior art but this alone may not make the structure toughened.

Also, any supporting layer comprising a cermet of a FeCr alloy in which a ceramic has been mixed in, should not be considered as a toughened ceramic. The somewhat increased mechanical integrity these support may offer is purely due to the adjusted thermal expansion among different layers of the cell as according to the prior art.

The inventions ceil is based on innovative support which are composed of three subcomponents; a toughened ceramic, a metal and a conductive material.

The ceramic may be PSZ and the metal may be chromium or aluminum containing steels, and the conductive material may i.e. be nickel, copper or Lanthanum-strontium-chromite. The purpose for using i.e.

chromium or aluminum containing alloys in the invention is for the metals characteristics neither to oxidize fully in oxidizing environment, nor reduce fully in reducing conditions within the life span and for typical operating conditions of the cell. Such cells may i.e. be FeCr, FeCrAi, FeCrAIY, NiCr, NiCrA! and IMiCrA!Y.

A toughened ceramic-metal has a throughout tougher supporting structure and due to its porous composition, it is significantly less brittle than i.e. the dense bipolar plates using Inconel.

The inventions toughened ceramic also allows for a faster start up time as the toughened ceramic is strong enough to withstand the thermal forces.

The support layer according to the present invention exhibits:

- higher fracture toughness compared to ceramic supports used in state- of-the-art CS-SOFC

- higher oxidation and degradation resistance compared to metallic

supports of MS-SOFC enabling that i) the sintering of successive layers can be conducted in air or in partial pressure of air and ii) reduce the operation related thermo-chemical degradation of the metallic support

- sufficient shrinkage during sintering of successive components that enables appropriate densification of the those components

Brief description of the drawings

In fig. 1 there is shown the steps of how a toughened ceramic substrate according to the present invention may be built up and how a fuel cell comprising the support layer disclosed above can be produced according to one specific embodiment of the present invention.

Specific embodiments of the invention

Below, some specific embodiments of the present invention are presented.

According to one embodiment, a diffusion barrier layer positioned in between the fuel electrode layer and the oxygen electrode layer. Examples of materials that may be used in the diffusion barrier layer is cerium oxide doped with gadolinium oxide, or samarium oxide etc.

According to another specific embodiment, the SOFC or SOEC layered structure is also impregnated with at least one metal, alloy or ceramic, or a mixture thereof. This impregnation component(s) acts as an electronic conducting material at cell operating temperatures and conditions. According to one specific embodiment, said SOFC layered structure is impregnated with at least one metal of Ni, Co, Cu, Fe, Pd, Pt, or Rh, or a ceramic of

La-i-xSrxCr-i.yMnyOs, Sri_-xTi_x O3 or Lai_-xSr_xTiO3, or a mixture thereof.

Furthermore, according to yet another embodiment of the present invention, the support layer is also free from any non-toughened conductive ceramic(s).

The support layer may comprise different material components.

According to one specific embodiment, the support layer comprises nickel- aluminium (Ni-AI), nickel-chromium (Ni-Cr), nickel-chromium-aluminium (Ni- Cr-AI), nickel-chromium-aluminium-yttrium (Ni-Cr-AI-Y), iron-aluminium (Fe- Al), iron-chromium (Fe-Cr), iron-chromium-aluminium (Fe-Cr-AI), iron- chromium-aluminium-yttrium (Fe-Cr-AI-Y), iron-nickel-aluminium (Fe-Ni-AI), titanium-aluminium (Ti-AI), titanium-aluminium-vanadium (Ti-AI-V), or a mixture thereof. According to the present invention, no pure nickel (Ni) or other pure metallic elements are used that, at cell operating temperature (between 600 and 800°C), oxidize readily and fully in air and reduce again completely in fuel atmosphere. Furthermore, for an iron-chromium (Fe-Cr) alloy, an alloy in which 12-32 wt% Cr is comprised is a suitable choice according to the present invention.

According to yet another specific embodiment, the support layer comprises PSZ (partially stabilized zirconia). In one embodiment, the support layer comprises PSZ stabilized with divalent or trivalent oxides. The PSZ can be zirconia stabilized with divalent (e.g. CaO, MgO) or trivalent (e.g. Y2O3) oxides in appropriate amounts to stabilize tetragonal phase. The amount of PSZ can vary between 5 to 100% out of the ceramic part in the support.

According to one specific embodiment of the present invention, the support layer comprises alumina (AI2O3) plus PSZ (partially stabilized zirconia). In relation to this it may be mentioned that alumina alone is not a toughened ceramic, however it becomes toughened once it makes a solid solution with PSZ.

Furthermore, in one specific embodiment, the support layer has 10-80 wt% toughened ceramic, when measured in the mixture of toughened ceramic and metal or alloy. According to one specific embodiment of the present invention, the support layer has 35-70 wt% toughened ceramic, when measured in the mixture of toughened ceramic and metal or alloy.

Besides the actual materials, also the shape and thickness or different layers and components may be relevant parameters. As an example, the form of materials in the substrate may include, either one or both types, of a) spherical or angular powders and b) whiskers in which one of the dimensions is significantly larger than the other two dimensions of the particle. Moreover, the thickness of the substrate may typically be in the range of from 50 pm to 1000 pm, preferably from 100 pm to 300 pm. Furthermore, the porosity of the substrate may be from 20 to 80 volume %, preferably from 30 to 50 volume%. To summarize, the support layer according to the present invention exhibits several advantages when compared to materials used today. As an example, when comparing one alternative according to the present invention with standard materials used today, the following points may be presented: - the fracture-resistance/ fracture toughness may be in the range of 3.5-4.5 MPa ^* m^{1 2} for the support layer according to the present invention (materials used today hold a value of 1 .5 - 1 .9 MPa ^* m^{1 2});

- toughness and durability with regards to frequent and severe thermal transients and cycle are increased;

- the start up time is reduced since the heat cycle may be faster due to the increased toughness;

- as no full redoxable metal is being used, the cell can withstand redox cycling; and

- increased mechanical strengths with regards to vibrations, mishandling and impacts.

The present invention is also directed to a method for producing a SOFC or SOEC layered structure, said method involving the impregnation of a support layer comprising:

- a metal or metal alloy or a mixture of several metals and/or metal alloys; and

- at least one toughened ceramic;

and being free from:

- any metal exhibiting full redox transformation at cell operating

temperature and conditions;

- any electro-catalytic ceramic or metal,

with at least one metal, alloy or ceramic, or a mixture thereof.

It should further be mentioned that the method for producing a SOFC or SOEC layered structure according to the present invention may either start as presented in fig. 1 below, that is by first building the support and then on top of that build a support layer, either on the anode or cathode side, or in producing the support layer first and then combine that with a substrate. This is further understood from below. Detailed description of the drawings

In fig. 1 there is shown one possible production method according to the present invention. In this case the starting point is based on a metallic reinforced toughened ceramic substrate comprising metal particles and ceramic particles. The substrate may be produced by premixing a toughened ceramic and metal powders as powder or as a slurry in suitable composition, such as e.g. 20 to 80 wt% ceramic, preferably 35 to 70 wt% ceramic. The substrate is then produced by pressing the powder or tape casting the slurry and the sintering. This is further explained below in relation to the description of the drawing.

Secondly, the supports are being anodized, more specifically the metallic particles in the substrate is anodized to create e.g. an aluminium oxide passive film.

The anodized ceramic substrate is then impregnated with metals such as nickel or copper and/or ceramics like LSCM or LST. The impregnated ceramic is then fired. This renders the support electronic conductor at operating temperatures. The impregnation is e.g. done by dip coating or by wet spraying.

Possible other SOFC functional layers will be introduced on top of the impregnated support.

The fuel cell is then constructed with an anode layer, an electrolyte layer, a diffusion barrier and a cathode layer. The support may be attached to the anode layers outside or to the cathode layers outside. The cell may have a diffusion layer or it may not have a diffusion layer. If the cell has a diffusion layer, it is positioned in between the fuel electrode layer and the oxygen electrode layer.

Below one type of cell composition according to one specific embodiment of the present invention is presented:

an anode layer of 10 to 30 pm in thickness and composed of

- 8YSZ + metal-catalyst, or

- LSCM + metal-catalyst, or

- LST+ metal -catalyst The metal catalyst can be e.g. Ni, Co, Pd, Pt, or Rh. The anode layer is produced by screen printing or tape casting or wet-spraying and sintered.

The electrolyte layer of an oxygen ion conducting material, such as 8YSZ or ScCeSZ or GDC, is coated on top of the anode by wet spraying, screen printing or tape casting, and is then sintered. Alternatively the layer is deposited by plasma spraying or PVD and no sintering follows.

A diffusion barrier layer and a cathode are then produced by

established methods.

Claims

1 . A solid oxide fuel cell (SOFC) or solid oxide electrolysis cell (SOEC) layered structure, said structure comprising:

- a fuel electrode layer;

- an oxygen electrode layer;

- a support layer being attached to the fuel electrode layer and/or the oxygen electrode layer, said support layer being composed of a porous

3D backbone structure and a coated material covering the 3D backbone structure;

c h a r a c t e r i z e d in that the 3D backbone structure of the support layer comprises:

- a metal or metal alloy or a mixture of several metals and/or metal

alloys; and

- at least one toughened ceramic;

and in that the 3D backbone structure of the support layer is free from:

- any metal exhibiting full reduction-oxidation (redox) transformation at cell operating temperature and conditions; and

- any electro-catalytic ceramic or metal.

2. The SOFC or SOEC layered structure according to claim 1 , a diffusion barrier layer positioned in between the fuel electrode layer and the oxygen electrode layer.

3. The SOFC or SOEC layered structure according to claim 1 , wherein the SOFC or SOEC layered structure also is impregnated with at least one metal, alloy or ceramic, or a mixture thereof.

4. The SOFC or SOEC layered structure according to any of claims 1 -3, wherein the support layer is also free from any non-toughened conductive ceramic(s).

5. The SOFC or SOEC layered structure according to any of claims 1 -4, wherein the support layer comprises nickel-aluminium (Ni-AI), nickel- chromium (Ni-Cr), nickel-chromium-aluminium (Ni-Cr-AI), nickel-chromium- aluminium-yttrium (Ni-Cr-AI-Y), iron-aluminium (Fe-AI), iron-chromium (Fe- Cr), iron-chromium-aluminium (Fe-Cr-AI), iron-chromium-aluminium-yttrium (Fe-Cr-AI-Y), iron-nickel-aluminium (Fe-Ni-AI), titanium-aluminium (Ti-AI), titanium-aluminium-vanadium (Ti-AI-V), or a mixture thereof.

6. The SOFC or SOEC layered structure according to any of claims 1 -5, wherein the support layer comprises PSZ (partially stabilized zirconia).

7. The SOFC or SOEC layered structure according to any of the preceding claims, wherein the support layer comprises PSZ stabilized with divalent or trivalent oxides.

8. The SOFC or SOEC layered structure according to any of the preceding claims, wherein the support layer comprises alumina (AI2O3) plus PSZ (partially stabilized zirconia).

9. The SOFC or SOEC layered structure according to any of the preceding claims, wherein the support layer has 10-80 wt% toughened ceramic, when measured in the mixture of toughened ceramic and metal or alloy.

10. The SOFC or SOEC layered structure according to any of the preceding claims, wherein the support layer has 35-70 wt% toughened ceramic, when measured in the mixture of toughened ceramic and metal or alloy.

1 1 . The SOFC or SOEC layered structure according to any of claims 3-10, wherein said SOFC layered structure also is impregnated with at least one metal of Ni, Co, Cu, Fe, Pd, Pt, or Rh, or a ceramic of La-i-xSrxCr-i.yMnyOs, Sr-i_-xTix O3 or Lai_-xSr_xTiO3, or a mixture thereof.