Patent Application
20100297527
Solid state electrochemical devices are well known in the art and include devices such as solid oxide fuel cells, electrolyzer cells, and the like. Devices commonly known as fuel cells comprise arrays of plates or tubes that directly convert to electricity (electric power) the energy released by oxidation of hydrogen. Simplistically, a fuel cell unit comprises layers, including an anode, a cathode, and an oxygen-permeable, dense electrolyte layer therebetween. Often such layers are supported by a rigid metal, ceramic, or cermet substrate.
Solid oxide fuel cell (SOFC) fabrication often involves co-sintering an electrolyte layer and a rigid support, which can be difficult due to differential shrinkage of the component materials, resulting in cracking, warping, delamination, breakage, and other forms of physical failure. Some examples of SOFCs are annular in shape, and are commonly referred to as tubular solid oxide fuel cells (TSOFC). In these types of SOFCs the active layers (anode, dense electrolyte, and cathode) may be placed on a porous metal support tube to complete the SOFC element. Other examples of SOFC are planar in shape where individual fuel cell elements are flat sandwiched layers of various materials comprising anode, dense electrolyte, and cathode.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.
The present invention is applicable to any configuration and/or shape of solid state electrochemical devices, including tubular, planar, tube sheet, etc. Representative examples are described herein with respect to a SOFC device.
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The porous support tube 11 may be prepared by conventional powder metallurgy techniques, such as molding, extrusion, casting, forging, isostatic compression, etc. The support tube 11 should be open on both ends.
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The first active fuel cell membrane layer 12 is an anode material, which can be any anode material, but is preferably comprised of a cermet composition. Examples of suitable cermet compositions include, but are not limited to Ni—YSZ, Ni—GdCeO2, Ni—SmCeO2, and Ag—SmCeO2. Anode thickness can be, for example, in a range of 3-100 μm. The anode can, for example, have an average pore size of0.3-50 μm and pore volume of 15-60 volume percent. The anode 12 is applied to the support tube 11 by a conventional method such as sol-gel, slurry, or wash coating, for example. The anode 12 can be sintered before or after the application of subsequent layers.
The next active fuel cell membrane layer 13 is a non-porous and/or operably dense O2-permeable or H2-permeable electrolyte composition. The terms “operably dense” and “operable density” as used herein mean that the electrolyte layer is sufficiently dense to be used in a fuel cell or electrolyzer, with minimal or no leakage of reactants therethrough. The skilled artisan will recognize that the terms “fully dense” and “full density” are also interpreted to have like meaning
Conventional electrolytes such as Yttria stabilized zirconia (YSZ) (>8 mole percent Y2O3 in ZrO2) and Gadolinium stabilized ceria (GSC) (>5 mole percent Gd2O3 in CeO2) require sintering temperatures in excess of 1300° C. for attain operable density (i.e., closed porosity). In accordance with the present invention, composite electrolytes comprised of mixtures of electrolytes and other oxides can be sintered to operable density at temperatures significantly less than 1300° C., minimizing the above-described differential shrinkage and low porosity of the final product, while allowing an operably dense electrolyte to be obtained. Moreover, a number of fast ion conducting oxides can be substituted into the compositions as described hereinbelow. Such substitutions can beneficially allow processing conditions to be tailored to match that of any support material used.
The electrolyte can, for example, have a thickness in a range of 2-300 μm. The electrolyte should be operably dense and gas tight to prevent the air and fuel from mixing. The electrolyte layer 13 may be deposited using a conventional method such as sol-gel, slurry, or wash coating, for example, and subsequently sintered.
The first two layers 12, 13 can be sintered simultaneously under either neutral (neutral as used herein means neither oxidizing nor reducing) or reducing conditions so that the anode maintains or attains the characteristics described hereinabove while achieving full densification of the electrolyte layer. The sintered electrolyte is preferably at least operably dense and essentially defect-free. Sintering parameters can include, for example, a temperature range of 1200-1300° C., preferably less than 1300° C., and a duration of 0.2 to 6 hours, usually about 1 to 2 hours.
The final layer is the cathode 14, which is generally comprised of alkaline earth substituted lanthanum manganite, alkaline earth substituted lanthanum ferrite, lanthanum strontium iron cobaltite, or a mixed ionic-electronic conductor, but the composition of the cathode 14 is not critical to the invention. The cathode 14 thickness can, for example, be in a range of 5-300 μm. The cathode 14 can, for example, have an average pore size of 0.3-50 μm and pore volume of 15-60 volume percent. The cathode 14 can also be deposited using a conventional method such as sol-gel, slurry, or wash coating, for example.
The final step is a sintering process that is composed of heating the entire TSOFC 10 in a neutral or reducing environment to 1000-1300° C., preferably less than 1300° C. for a duration of 0.2 to 6 hours, usually about 1 to 2 hours, depending on the cathode material used. The term neutral as used herein means neither oxidizing nor reducing.
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In some accordance with the present invention, active fuel cell membrane layers can be deposited and sintered as described hereinabove in a planar support to form a planar SOFC. The skilled artisan will recognize that any shape and configuration of the support can be employed to make any desired shape and configuration SOFC.
For the sintering steps described hereinabove, sintering temperatures below 1300° C. are desirable in order to minimize interdiffusion of electrolyte layers with other layers of a SOFC structure. Moreover, a sintering temperature below 1300° C. is desirable in order to minimize sintering, densification, and/or melting of other, non-electrolyte layers of a SOFC structure. When sintered at 1300° C., conventional SOFCs had extremely low porosity of the final SOFC element. However, for applications described herein the sintering temperature of the electrolyte needs to be reduced to less than 1300° C.
Examples of composite electrolyte materials of the present invention include composites comprising 8YSZ in a range of 50-95 wt. %, balance 3YSZ. Either the 8YSZ or the 3YSZ can be substituted with a fast ion conducting oxide having the general formula A1-x-yBxCy where:
Examples of rare-earth elements (RE) include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
The composites allow the sintering temperature of Zr0.84Y0.16O2 to be reduced by up to 100° C. to achieve full density in thin film greater than or equal to 1 micron meter when applied by any thin film method.
In order to test various contemplated electrolyte formulations, 3YSZ powder, 8YSZ powder, 10YSZ powder, and mixtures of 3YSZ powder and 8YSZ powder were prepared in the following percentages: 90% 8YSZ/10% 3YSZ, 80% 8YSZ/20% 3YSZ, 70% 8YSZ/30% 3YSZ, and 60% 8YSZ/40% 3YSZ. Each of the mixtures was pressed into a pellet using a uniaxial die, followed by isostatic pressing of the pellet to increase green density. The resulting pellets were sintered for 1 hour at temperatures shown in
Alternatively, formulation and heat treatment temperatures can be altered to be able to process at other temperature conditions or achieve different properties, such as mechanical strength or conductivity.
A powder mixture comprising 70% 8YSZ/30% 3YSZ is combined with appropriate conventional solvent and dispersant and mixed in a ball mill. The resulting slurry is used to coat surface of a SOFC with green electrolyte coating during manufacture as described hereinabove. The coated SOFC is sintered at 1273° C. for 1 hour in Ar—4% H2 to a density of 93% theoretical density.
As an example of the desired density that may be achieved in practicing the present invention,
The skilled artisan will recognize that, for additional coating process functionalities, coating green strength, and/or final product properties, at least one optional, conventional binder, dispersant, plasticizer, and/or rheology modifier can be added to the mixtures.
While there has been shown and described what are at present considered to be examples of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/179,513 filed on May 19, 2009, the entire disclosure of which is incorporated herein by reference. U.S. patent application Ser. No. 11/755,945 entitled “Solid Oxide Fuel Cell Having Internal Active Layers” filed on May 31, 2007 by Timothy R. Armstrong, Roddie R. Judkins, Beth L. Armstrong, and Brian L. Bischoff is specifically referenced and incorporated herein by reference.
The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. This invention arose under Cooperative Research and Development Agreement No. ORNL-0720.0 between UT-Battelle, LLC and Worldwide Energy, Inc.
| Number | Date | Country | |
|---|---|---|---|
| 61179513 | May 2009 | US |
