FORMATION OF SOLID OXIDE FUEL CELLS

Abstract

A method of producing a solid oxide fuel cell comprising tape casting an anode support and spraying layers onto the anode support. The layers that can be sprayed onto the anode support include an anode functional layer, an electrolyte layers, and a cathode functional layer.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

A method of forming solid oxide fuel cells.

BACKGROUND OF THE INVENTION

The world relies heavily on energy produced from fossil fuels, but as a non-renewable energy source (at least in our lifetimes), fossil fuels have serious limitations. The ever-increasing demand and dwindling supply of fossil fuels will inevitably cause significant problems in the future. In remote areas of developing countries, transmission and distribution of fossil fuel-generated energy can be difficult and expensive. Additionally, the burning of fossil fuels results in the formation of smog and global warming, and further contributes to our environmental problems. Thus, developing a clean alternative energy industry is key to improving the quality of life for individuals and communities, and to ameliorate global warming and other environmental problems.

The critical technical barrier to the widespread application of various alternative energy technologies is performance limitations of the key materials. For example, in solar-fuel production, the efficiency is relatively low since the current photocathode materials show sluggish H₂ evolution reaction kinetics and the photoanode materials have insufficient light absorption and carrier collection capabilities. In wind power, the lifetime of turbine blades currently made of polymer-matrix composite materials reinforced with fiberglass or graphite fibers can be further enhanced when a new material with adequate stiffness to prevent failure as well as sufficient long term fatigue in harsh conditions is developed. Therefore, materials science and technology plays a pivotal role in building the world's energy future, from fundamental discovery science, to improving energy production processes. The discovery and optimization of new materials could effectively advance solutions to our energy challenges.

The demand for clean, secure, and renewable energy has stimulated great interest in fuel cells. Fuel cells are one distinct category of devices that are capable of converting chemical energy into electrical energy. Among the fuel cells that are currently under active development, alkaline, polymeric-electrolyte-membrane and phosphoric-acid fuel cells all require essentially pure hydrogen as the fuel to be fed to the anode.

Solid Oxide Fuel Cells (“SOFCs”), on the other hand, are a type of fuel cells that use a solid oxide or ceramic as the electrolyte of a cell. The basic solid oxide fuel cell is generally made up of three layers. A single cell consisting of these three layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an “SOFC stack”. The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 500 to 1,000° C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again.

SOFCs offer great promise for the most efficient and cost-effective utilization of a wide variety of fuels such as hydrocarbons, coal gas and gasified biomass. Because of the relatively high operating temperature (500-1000° C.), the fuel processing reaction can be carried out within the cell stacks without additional fuel processors. Another advantage of SOFCs is the fuel flexibility. A wide variety of practical hydrocarbons such as methane, propane, gasoline, diesel and kerosene can be directly utilized as the fuels in SOFCs. The direct utilization of hydrocarbon fuels will increase the operating efficiency and reduce system costs, which will accelerate substantially the use of SOFCs in transportation, residential and distributed-power application. Among the hydrocarbon fuels, natural gas such as methane is regarded as relatively cheap and popularly available fuel with plenty of deposits. Additionally, SOFCs that can directly run on natural gas would highly reduce the operating cost and accelerate the commercialization of SOFC system.

The basic chemical reactions at the anode side of an SOFC is the oxidation of fuels, such as hydrogen gas and/or carbon monoxide, to generate electrons:

Anode: H₂+O²⁻→H₂O+2e⁻

and/or

CO+O²⁻→CO₂+2e⁻

The reaction at the cathode side is the reduction of oxygen to oxygen ions:

Cathode: O₂+4e⁻→2O²⁻

Therefore, the overall reaction of an SOFC becomes:

Overall: H₂+½O₂→H₂O

Or

H₂+CO+O₂→H₂O+CO₂

Therefore, SOFCs typically run on pure hydrogen or mixture of hydrogen and carbon monoxide by internally or externally reforming a hydrocarbon fuel, while air serves as the oxidant. As shown above, if pure hydrogen is used, then the product is pure water, whereas carbon dioxide is produced if carbon monoxide is also used.

In an effort to reduce fuel electrode manufacturing costs, sintering processes have been attempted, such as those described in U.S. Pat. Nos. 4,971,830, 5,035,962, 5,908,713 and 6,248,468. However, fuel electrodes applied by a sintering process are relatively time consuming in that it still requires at least two processing steps, an initial application followed by high temperature sintering. Moreover, sintered fuel electrodes may experience marginal physical stability over time.

Other attempts to reduce fuel electrode fabrication costs include plasma spraying (e.g. atmospheric plasma spraying “APS”, vacuum plasma spraying “VPS”, plasma arc spraying, flame spraying) which generally involves spraying a molten powdered metal or metal oxide onto an underlying substrate surface using a plasma thermal spray gun to form a deposited layer having a microstructure generally characterized by accumulated molten particle splats. Plasma spraying techniques are described in U.S. Pat. Nos. 3,220,068, 3,839,618, 4,049,841, and U.S. Pat. Nos. 3,823,302 and 4,609,562 generally teach plasma spray guns and use thereof, each of which are herein incorporated by reference in their entirety. Although plasma spraying has been used for fabrication of certain fuel cell layers, such as those described in U.S. Pat. Nos. 5,085,742, 5,085,742, 5,234,722 5,527,633 (plasma sprayed electrolyte) U.S. Pat. No. 5,426,003 (plasma sprayed interconnect), U.S. Pat. No. 5,516,597 (plasma sprayed interlayer) and U.S. Pat. No. 5,716,422 (plasma sprayed air electrode), use of such plasma spraying techniques have been of limited value when used to apply a fuel electrode onto an electrolyte because they tend to result in a fuel electrode that poorly adheres to the electrolyte and exhibits poor thermal cyclability due to the mismatch of thermal coefficients of expansion between the metal portion of the fuel electrode and the ceramic electrolyte. Moreover, these conventional plasma spraying techniques tends to result in a fuel electrode that has a low porosity after continued use, thereby causing voltage loss when current flows as a result of polarization due to a low rate of diffusion of fuel gases into and reaction product out from the interface between the fuel electrode and electrolyte.

There is thus a need for a SOFC and a method for making the SOFC that can generally achieve above-described favorable technical properties and can be manufactured at a low cost.

BRIEF SUMMARY OF THE DISCLOSURE

A method of forming a solid oxide fuel cell comprising tape casting an anode support and spraying layers such as an anode functional layer, electrolyte layers, and a cathode functional layer onto the anode support.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a solid oxide fuel cell cross section.

FIG. 2 depicts a solid oxide fuel cell cross section.

FIG. 3 depicts a fracture cross sectional image of a spray coating technique.

FIG. 4 depicts a graph of film thickness versus number of spray coater passes.

FIG. 5 depicts a graph of electrochemical performance of a solid oxide fuel cell.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

The present embodiments describe a method of forming a fuel cell by tape casting an anode support and spraying layers such as an anode functional layer, electrolyte layers and a cathode functional layers onto the anode support.

In one embodiment of the SOFC the anode support and the anode functional layer is typically porous to allow the fuel to flow towards the electrolyte. Anodes are typically chosen for their (1) high electrical conductivity; (2) a thermal expansion that matches those of the adjoining components; (3) the capacity to avoid coke deposition; (4) fine particle size; (5) chemical compatibility with another cell components (electrolyte and interconnector) under a reducing atmosphere at the operating temperature; (6) large triple phase boundary; (7) high electrochemical or catalytic activity for the oxidation of the selected fuel; (8) high porosity (20 -40%) adequate for the gas phase transport of the fuel and reaction products ; and (9) electronic and ionic conductive phases. In the current SOFC any known anode electrodes can be utilized. Types of anodes that can be used include Ni/YSZ, Cu/Ni, perovskite structures with a general formula of ABO₃. In the perovskite structure the A cations can be group 2, 3, or 10 elements or more specifically cations such as, La, Sr, Ca or Pb. Also in the perovskite structure the B cations can be group 4, 6, 8, 9, or 10 elements or more specifically cations such as Ti, Cr, Ni, Fe, Co or Zr. Other materials that the anode could be include nickel oxide, nickel, yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate, or combinations thereof.

In one embodiment the anode can be pre-reduced at a temperature from about 400° C. to about 800° C. in a reducing atmosphere containing 1-100% hydrogen or other reducing gas atmospheres.

Tape casting of the anode support can be done by preparing an anode slurry, degassing the anode slurry and casting the anode slurry onto a support to form a ceramic tape. The tape is then dried to form the anode support. In one embodiment the tape casting occurs at temperatures ranging from about −50° C. to about 50° C. or from 5° C. to about 50° C. The thickness of the anode support can range from about 50 μm to about 1 mm. A cross section of a porous anode support prepared by tape casting is shown in FIG. 1. FIG. 1 depicts a cross section of a solid oxide fuel cell with a Ni-yttrium zirconium oxide support fabricated by tape casting, a Ni-scandium-zirconium oxide anode functional layer applied by spray coating, bi-layer electrolyte comprised of scandium-zirconium oxide and gadolinium cerium oxide applied by spray coating, and a strontium samarium cobalt oxide cathode-gadolinium cerium oxide cathode applied by spray coating.

In one embodiment of the SOFC the cathode functional layer is typically porous to allow the oxygen reduction to occur. Any cathode material known to those skilled in the art can be used. One example of cathode materials that are typically used include perovskite-type oxides with a general formula of ABO₃. In this embodiment the A cations can be lower valance cations such as La, Sr, Ca or Pb. The B cations can be metals such as Ti, Cr, Ni, Fe, Co or Zr. Examples of these perovskite-type oxides include LaMnO₃. In one differing embodiment the perovskite can be doped with a group 2 element such as Sr²⁺ or Ca²⁺. In another embodiment cathodes such as Pr_0.5Sr_0.5FeO₃; Sr_0.9Ce_0.1Fe_0.8Ni_0.2O₃; Sr_0.8Ce_0.1Fe_0.7Co_0.3O₃; LaNi_0.6Fe_0.4O₃; Pr_0.8Sr_0.2Co_0.2Fe_0.8O₃; Pr_0.7Sr_0.3Co_0.2Mn_0.8O₃; Pr_0.8Sr_0.2FeO₃; Pr_0.6Sr_0.4Co_0.8Fe_0.2O₃; Pr_0.4Sr_0.6Co_0.8Fe_0.2O₃; Pr_0.7Sr_0.3Co_0.9Cu_0.1O₃; Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃; Sm_0.5Sr_0.5CoO₃; or LaNi_0.6Fe_0.4O₃ can be utilized. Other materials that the cathode could be include lanthanum strontium iron cobalt oxide, doped ceria, strontium samarium cobalt oxide, lanthanum strontium iron oxide, lanthanum strontium cobalt oxide, barium strontium cobalt iron oxide, or combinations thereof. A samarium strontium cobalt oxide-gadolinium cerium oxide cathode applied by spray coating is shown in FIG. 2. FIG. 2 depicts a cross section of a solid oxide fuel cell with a Ni-yttrium zirconium oxide support fabricated by tape casting, a Ni-scandium-zirconium oxide anode functional layer applied by spray coating, bi-layer electrolyte comprised of scandium-zirconium oxide and gadolinium cerium oxide applied by spray coating, and a strontium samarium cobalt oxide cathode-gadolinium cerium oxide cathode applied by spray coating.

The electrolyte layer used in the SOFC is responsible for conducting ions between the electrodes, for the separation of the reacting gases, for the internal electronic conduction blocking, and for forcing the electrons to flow through the external circuit. Some of the typical characteristics that electrolytes typically invoke include (1) an oxide-ion conductivity greater than 10⁻² S.cm⁻¹ at the operating temperature; (2) negligible electronic conduction, which means an electronic transport number close to zero; (3) high density to promote gas impermeability; (4) thermodynamic stability over a wide range of temperature and oxygen partial pressure; (5) thermal expansion compatible with that of the electrodes and other cell materials from ambient temperature to cell operating temperature; (6) suitable mechanical properties, with fracture resistance greater than 400 MPa at room temperature; (7) negligible chemical interaction with electrode materials under operation and fabrication conditions to avoid formation of blocking interface phases; (8) ability to be elaborated as thin layers (less than 30 μm) and (9) low cost of starting materials and fabrication.

In the current SOFC the electrolyte can be any electrolyte known to those skilled in the art. In one embodiment the electrolyte is a dense stabilize zirconia or a doped ceria. In one embodiment the electrolyte comprises a porous BZCYYb as the backbone and carbonate as the secondary phase within the pores of

The weight ratio of BZCYYb in the composite electrolyte may vary, as long as the composite electrolyte can reach higher conductivity as well as current density as compared to non-composite electrolyte. In one embodiment, the weight ratio of BZCYYb in the composite electrolyte ranges from 9:1 to 1:1, but more preferably ranges from 50-90% or 70-80%. In another embodiment, the weight ratio of BZCYYb is about 75%.

The weight percentage of carbonate in the composite electrolyte also may vary, as long as the composite electrolyte can maintain physical integrity during operation. In one embodiment, the weight percentage of carbonate in the composite electrolyte ranges from 10 to 50 wt %. In another embodiment, the weight percentage of carbonate in the composite electrolyte ranges from 20 to 30 wt %, in yet another embodiment, the carbonate is about 25%.

In one example of preparing BZCYYb lithium-potassium carbonate is typically made first. Stoichiometrical amount of Li₂CO₃ and K₂CO₃ were mixed in the weight proportion of 45.8:52.5 and milled in a vibratory mill for 1 hour. The mixture was then heated to 600° C. for 2 hours. The heated mixture was then quenched in air to the room temperature and ground. The resulting lithium-potassium carbonate was used later in the preparation of composite electrolyte with BZCYYb.

In one embodiment the BZCYYb powder was prepared by solid-state reaction, but other methods could also be used. Stoichiometric amounts of high-purity barium carbonate, zirconium oxide, cerium oxide, ytterbium oxide and yttrium oxide powders (all from Sigma-Aldrich® Chemicals) were mixed by ball milling in ethanol (or other easily evaporated solvent) for 24 h, followed by drying at 80° C. for overnight and calcinations at 1100° C. in air for 10 h. The calcinated powder was ball milled again, followed by another calcination at 1100° C. in air for 10 h to produce single phase BZCYYb.

The resulted BZCYYb powder and the carbonate obtained above were mixed at weight ratio of 75:25 and thoroughly ground again for one hour. The mixture was then heated to 680° C. for 60 minutes until only the carbonate melted and wet the BZCYYb grain boundaries in the mixture. Next, it was quenched (i.e. fast cooling) in air to room temperature. The quenched mixture was ground again to get the composite electrolyte powder.

In another example an alternate way of preparing BZCYYb powder can be described. In this embodiment stoichiometric amounts of high-purity barium carbonate, zirconium oxide, cerium oxide, ytterbium oxide, and yttrium oxide powders (all from Sigma-Aldrich® Chemicals) were mixed by ball milling in ethanol for 48 h, followed by drying in an oven and calcination at 1100° C. in air for 10 h. The calcined powder was ball milled again, followed by another calcination at 1100° C. in air for 10 h.

The CeO₂ and ZrO₂ powders with different particle sizes were used to optimize the fabrication procedures. To prepare electrolyte samples for the conductivity measurement, we pressed the calcined powders isostatically into a disk at 274.6 MPa. The green disks had a diameter of 10 mm, with a typical thickness of 1 mm. The disks were then sintered at 1500° C. for 5 h in air (relative density>96%).

In some embodiments a Sc-doped BZCY powder can be prepared. In one example of this embodiment BZCY—Sc with a nominal composition of BaCe_0.7Zr_0.1Y_0.1Sc_0.1O_3-δ (BZCY—Sc) was synthesized by a conventional solid state reaction (SSR) method. Stoichiometric amount of high-purity barium carbonate, zirconium oxide, cerium oxide, yttrium oxide and scandium oxide powders (BaCO₃:ZrO2:CeO₂:Y₂O₃:Sc₂O₃=167.33:12.32:120.48:22.58:13.79, all from Sigma-Aldrich® Chemicals) were mixed by ball milling in ethanol for 24 hours, followed by drying at 80° C. for overnight and calcinations at 1100° C. in air for 10 hours. The calcined powder was ball milled again, followed by another calcination at 1100° C. in air for 10 hours to produce single phase BZCY—Sc.

In the present method, the calcining step is carried out at preferably higher than 1000° C. in air for 10 hours. However, the temperature and the length of calcination can vary, depending on different factors to be considered, such as the particle size chosen. The particle size of the zirconium oxide powder is preferably between 50 nm and 200 nm, and more preferably between 50 nm and 100 nm. The particle size of the cerium oxide powder is preferably between 1 μm and 20 μm, and more preferably between 5 and 10 μm.

The different layers of the process can by sprayed by preparing a ceramic slurry following by delivering the slurry to a spray nozzle. The slurry is then sprayed and atomized onto an anode support to form a sprayed layer which is dried. The flow rate of the spraying can range from about 0.1 ml/min to about 20 ml/min. The pressure of the spraying can range from about from about 0.5 psi to about 100 psi. The atomization of the spray can either be ultrasonic or pneumatic.

The layers or each successive layer added by the spraying can be identical to the one before it or different. In a more specific example the material used for the anode support could be different or identical to the material used of the anode functional layer. In one embodiment the thickness of each layer deposited by single spraying pass ranges can range from about 50 nm to about 1 μm. In one embodiment each layer sprayed by this method is repeated at least two times. In another embodiment each layer sprayed by this method is repeated at least three times. In one embodiment, the spraying of layers is repeated till the cumulative thickness of the layers on top of the tape casted anode is at least 1 μm. FIGS. 3 and 4 shows an example of thickness control with the spray coating technique. FIG. 3 depicts fracture cross sectional scanning electron microscope images of the thickness using 1, 2, 3 and 4 spray passes. FIG. 4 graphs the film thickness versus number of spray coater passes. In another embodiment the thickness of the deposited layers has a variance of less than one sigma.

In one embodiment a heat treatment can be applied after the spraying of the different layers of the anode support. The temperature of the heat treatment can range between 850° C. to about 1500° C. The electrochemical performance of a solid oxide fuel cell with a Ni-based anode support fabricated by tape casting, a Ni-scandium-zirconium oxide anode functional layer applied by spray coating, bi-layer electrolyte comprised of scandium-zirconium oxide and gadolinium cerium oxide applied by spray coating, and a strontium samarium cobalt oxide cathode-gadolinium cerium oxide cathode applied by spray coating is shown in FIG. 5. The cell was characterized at an operating temperature of 650° C. and a fuel of humidified hydrogen.

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

Claims

1. A method of producing a solid oxide fuel cell comprising: tape casting an anode support; andspraying layers comprising: an anode functional layer, an electrolyte layer and a cathode functional layer onto the anode support.

2. The method of claim 1, wherein the process of tape casting the anode support comprises: preparing an anode slurry;degassing the anode slurry;casting the anode slurry onto a support to form a ceramic tape; anddrying the ceramic tape to produce the anode support.

3. The method of claim 1, wherein the process of spraying layers comprises: preparing a ceramic slurry;delivering the slurry to a spray nozzle;spraying and atomizing the slurry onto an anode support to form a sprayed layer; anddrying the sprayed layer.

4. The method of claim 1, wherein the tape casting occurs at a temperatures from about −50° C. to about 50° C.

5. The method of claim 1, wherein the anode support arranges in thickness from about 50 μm to about 1 mm.

6. The method of claim 1, wherein the spraying occurs at a temperature ranges from about 5° C. to about 50° C.

7. The method of claim 1, wherein a heat-treatment can be applied after spraying.

8. The method of claim 7, wherein the temperature of the heat-treatment ranges from about 850° C. to about 1500° C.

9. The method of claim 1, wherein flow rate of the spraying ranges from about 0.1 ml/min to about 20 ml/min.

10. The method of claim 1, wherein the pressure of the spraying ranges from about 0.5 psi to about 100 psi.

11. The method of claim 1, wherein the thickness of the each layer deposited by single spraying pass ranges from about 50 nm to about 1 μm.

12. The method of claim 3, wherein the atomization is either ultrasonic or pneumatic.

13. The method of claim 1, wherein the spraying of layers is repeated at least two times.

14. The method of claim 1, wherein the spraying of layers is repeated at least three times.

15. The method of claim 1, wherein the spraying of layers is repeated till the cumulative thickness of the layer on top of the tape casted anode support is at least 1 μm.

16. The method of claim 1, wherein each layer sprayed is different than all subsequent layers.

17. The method of claim 1, wherein the thickness of the deposited layers has a variance of less than one sigma.

18. The method of claim 1, wherein the materials used for the anode support is different from the materials used for the anode functional layer. The method of claim 1, wherein the materials used for the anode support are the same as the materials used for the anode functional layer.

19. The method of claim 1, wherein the anode functional layer is selected from a mixture comprising an electronic conductor and an ionic conductor.

20. The method of claim 20, wherein the electronic conductor is selected from the group consisting of: NiO, CO-oxide, CuO or combinations thereof.

21. The method of claim 20, wherein the ionic conductor is selected from the group consisting of: yittria stabilized zirconia, scandia stabilized zirconia, gadolinium doped ceria, samarium doped ceria, doped barium zirconate cerate or combinations thereof.

22. The method of claim 1, wherein the cathode functional layer is selected from a mixture comprising an electronic conductor and an ionic conductor.

23. The method of claim 23, wherein the electronic conductor is selected from the group comprising: lanthanum strontium iron cobalt oxide, strontium samarium cobalt oxide, lanthanum strontium iron oxide, lanthanum strontium cobalt oxide, baraium strontium cobalt iron oxide or combinations thereof.

24. The method of claim 23, wherein the ionic conductor is selected from the group comprising: doped ceria, stabilized zirconia and combinations thereof.

25. The method of claim 1, wherein the electrolyte layer is a dense stabilized zirconia.

26. The method of claim 1, wherein the electrolyte layer is a doped ceria.

27. The method of claim 1, wherein the electrolyte layer is a porous BZCYYb electrolyte.

28. The method of claim 1, wherein the electrolyte layer is a Sc doped BZCY.