Patent Application
20080299436
The invention Is related to a composite ceramic electrolyte. The invention is also related to a method of forming a composite ceramic electrolyte, and devices made therefrom.
Solid oxide fuel cells (SOFCs) are promising devices for producing electrical energy from fuel with high efficiency and low emissions. One barrier to the widespread commercial use of SOFCs is the high manufacturing cost. The manufacturing cost is largely driven by the need for state-of-the-art ceramic anodes, cathodes, or electrolytes, which allow the fuel cells to operate at high temperatures (e.g., about 800° C.). Fuel cell components that can meet these criteria require materials of construction that can be expensive to manufacture. Solid oxide fuel cells need to have high power densities and fuel utilizations, and need to be large in size, in order to make the technology economically feasible.
Thermal spray processes, such as air plasma spray, have the potential to provide large-area cells on interconnect supports that may reduce manufacturing costs. However, air-plasma-sprayed coatings typically contain both pores and microcracks, which in the case of a ceramic electrolyte may provide leak paths for the fuel and air. Microcracks of this type are typically formed at interlamellar splat boundaries during deposition, or are formed through the thickness of the coating, due to large thermal expansion strains caused during deposition. Such defects may limit the open cell voltage and fuel utilization. Therefore, there is a continuous need to improve the performance of a ceramic electrolyte.
The present invention meets these and other needs by providing a composite ceramic electrolyte having substantially reduced permeability.
One embodiment of the invention is a composite ceramic electrolyte. The composite ceramic electrolyte has a microstructure, which comprises a first ceramic composition comprising a plurality of nano-dimensional microcracks; and a second ceramic composition substantially embedded within at least a portion of the plurality of nano-dimensional microcracks. The first and the second compositions are different from each other.
Another embodiment is a solid oxide fuel cell. The solid oxide fuel cell comprises an anode; a cathode; and a composite ceramic electrolyte disposed between the anode and the cathode. The composite ceramic electrolyte has a microstructure, which comprises a first ceramic composition comprising a plurality of nano-dimensional microcracks; and a second ceramic composition substantially embedded within at least a portion of the plurality of nano-dimensional microcracks, wherein the first and the second compositions are different.
In another embodiment, the invention provides a method of forming a composite ceramic electrolyte. The method comprises the steps of providing, a first ceramic composition comprising a plurality of nano-dimensional microcracks; and closing a number of the nano-dimensional microcracks with a second ceramic composition, wherein the first and the second compositions are different; so as to form a composite ceramic electrolyte having a microstructure which comprises a first ceramic composition comprising a plurality of nano-dimensional microcracks and a second ceramic composition substantially embedded within at least a portion of the plurality of nano-dimensional microcracks.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawing.
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” “first,” “second,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular aspect of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the aspect may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
As used herein, “a nano-dimensional microcrack” is meant to describe a microcrack with at least one of the dimensions (length, width, or breadth) in the nanometer range. As used herein, a microcrack is meant to encompass any kind of crack, crevice, or an opening of any shape. In the following embodiments, nano-dimensional microcracks typically have an average width less than about 200 nanometers, and an average length less than about 2000 nanometers.
One embodiment of the invention is a composite ceramic electrolyte.
In these embodiments, the composite ceramic electrolyte is in the form of a monolithic structure. A “monolithic structure” as used herein, means a three-dimensional body portion constituting a single unit without a joint. This is in contrast to a body formed of multiple components, such as a laminated structure, or a multi-layered structure. The monolithic structure that does not have an inherent interface is expected to be substantially free of delamination problems. Delamination may lower the electrolyte ionic conductivity,
The microstructure of the as-deposited first ceramic composition, including dimensions of the microcracks and porosity of the electrolyte, depends mainly on the deposition technique and processing conditions. In one embodiment, the nano-dimensional microcracks have an average microcrack width of less than about 200 nanometers. In another embodiment, the nano-dimensional microcracks have an average microcrack length of less than about 2000 nanometers. (Both dimensional attributes can be present in a single microstructure as well). The microcrack dimensions may he tuned by adjusting the processing parameters, as known in the art. Typically, the plurality of nano-dimensional microcracks has, on average, an aspect ratio of at least about 4. In a specific embodiment, the plurality of nano-dimensional microcracks has, on average, an aspect ratio in the range from about 8 to about 12. Typically, the as-deposited first ceramic composition layer has a porosity of more than about 5 volume percent. The composite electrolyte typically has a porosity less than the as-deposited first ceramic composition layer. In one embodiment, the composite electrolyte has a porosity of less than about 5 volume percent. In another embodiment, the porosity is less, than about 2 volume percent.
The composition of the composite ceramic electrolyte, in part, depends on the end-use application. When the composite ceramic electrolyte is used in a solid oxide fuel cell, or an oxygen- or synthesis gas generator, the electrolyte may be composed of a material capable of conducting ionic species (such as oxygen ions or hydrogen ions), yet may have low electronic conductivity. When the composite ceramic, electrolyte, is used in a gas separation device, the composite ceramic electrolyte may be composed of a mixed ionic electronic conducting material. In all the above embodiments, the electrolyte may be desirably gas-tight to electrochemical reactants.
With reference to
In the case of an electrolytic oxygen separation device, oxygen is driven across the membrane by applying a potential, difference and supplying energy. In such embodiments, the first ceramic composition 22 is usually chosen from electrolytes well known in the art, such as yttria-stabilized zirconia (e.g., (ZrO2)0.92(Y2O3)0.08, YSZ), scandia-stabilized zirconia (SSZ), doped ceria such as (CeO2)0.8(Gd2O3)0.2 (CGO), doped lanthanum gallate such as La0.8Sr0.2Ga0.85Mg0.15O2.285 (LSGM), and doped bismuth oxide such as (Bi2O3)0.75(Y2O3)0.25, and the like.
In the case of a gas separation device, where partial pressures, rather than applied potential, are used to move ions across the electrolyte, the first ceramic composition 22 is often a mixed ionic electronic conductor (MIEC). Non-Limiting examples of mixed ionic electronic conductor are La1-xSrxCoO3-8; (2≧×≧0.10)(LSC), SrCo1-xFexO3-5;(0.3≧×≧0.20), La0.6Sr0.4Co0.6Fe0.4O3.8; LaNi0.6Fe0.4O3, and Sm0.5Sr0.5CoO3.
Typically, the second ceramic composition 26 comprises an oxide. In some embodiments, the oxide is selected from the group consisting of a rare-earth oxide, a transition metal oxide, and an alkaline earth metal oxide, in certain particular embodiments, the oxide Is selected from the group consisting of alumina, bismuth oxide, ceria, lanthanum gallate, silica, hafnia, thoria, zirconia, yttria, calcium oxide, gadolinium oxide, samarium oxide, and europium oxide. In an exemplary embodiment, the second ceramic composition 26 comprises gadolinium-doped ceria.
According to the embodiments of the invention, it was discovered that the permeability of the ceramic electrolyte is significantly reduced when the second ceramic composition 26 is incorporated into the nano-dimensional microcracks 24. Permeability of the composite electrolyte 20 may be in part controlled by the extent of the microcrack filling. Accordingly, in certain embodiments, at least one of nano-dimensional microcracks is at least partially embedded with a second ceramic composition 26. In certain specific embodiments, at least some of the nano-dimensional microcracks may be embedded with the second ceramic composition 26, and in other embodiments, substantially all of the microcracks are embedded with the second ceramic composition 26. In certain embodiments, at least about 25 volume percent of the nano-dimensional microcracks are embedded with the second ceramic composition 26 (i.e., measured as a percentage of the total volume of all of the cracks). In other situations, at least about 50 volume percent of the nano-dimensional microcracks are embedded. In some instances, about 25 volume percent to about 75 volume percent of the nano-dimensional microcracks are embedded with the second ceramic composition (26).
Typically, the composite ceramic electrolyte 20 comprises less than about 10 volume percent of the second ceramic composition 26, based on the total volume of the composite ceramic electrolyte. The amount of the second ceramic composition 26 present is usually in a range from about 1 volume percent to about 6 volume percent, based on the total volume of the composite ceramic electrolyte 20. Based in part on the teachings herein, one skilled in the art would know how to optimize the composition of the components, and their volume fractions, depending on the device structure and operation conditions.
Another embodiment of the invention is a solid, oxide fuel cell (SOFC). A fuel cell is an energy conversion device that produces electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. As shown in
The interconnect portion 32 defines a plurality of airflow channels 44 in intimate contact with the cathode 34, and a plurality of fuel flow channels 46 in intimate contact with the anode 36 of an adjacent cell repeat unit 40, or vice versa. During operation, a fuel flow 48 is supplied to the fuel flow channels 46. An airflow 50, typically heated air, is supplied to the airflow channels 44. Interconnects 32 and 33 may he constructed in a variety of designs, and with a variety of materials. Typically, the interconnect is made of a good electrical conductor such as a metal or a metal alloy. The interconnect desirably provides optimized contact area with the electrodes.
In the exemplary embodiment shown in
The main purpose of the anode layer 36 is to provide reaction sites for the electrochemical oxidation of a fuel introduced into the fuel cell. In addition, the anode material is desirably stable in the fuel-reducing environment, and has adequate electronic conductivity, surface area and catalytic activity for the fuel gas reaction under operating conditions. The anode material desirably has sufficient porosity to allow gas transport to the reaction sites. The anode layer 36 may be made of any material having these properties, including but not limited to, noble metals, transition metals, cermets, ceramics and combinations thereof. Non-limiting examples of the anode layer material include nickel, nickel alloy, cobalt, Ni—YSZ cermet, Cu—YSZ cermet, Ni—Ceria cermet, or combinations thereof. In certain embodiments, the anode layer comprises a composite of more than one material.
The cathode layer 34 is typically disposed adjacent to the composite electrolyte 38. The main purpose of the cathode layer 34 is to provide reaction sites for the electrochemical reduction of the oxidant. Accordingly, the cathode layer 34 is desirably stable in the oxidizing environment, has sufficient electronic and ionic conductivity, has a surface area and catalytic activity for the oxidant gas reaction at the fuel cell operating conditions, and has sufficient porosity to allow gas transport to the reaction sites. The cathode layer 34 may be made of any materials meeting these properties, including, but not limited to, an electrically conductive, and in some cases ionically conductive, catalytic oxide such as, strontium doped LaMnO3, strontium doped PrMnO3, strontium doped lanthanum ferrites, strontium doped lanthanum cobaltites, strontium doped lanthanum cobaltite ferrites, strontium ferrite, SrFeCo0.5Ox, SrCo0.8Fe0.2O3-8; La0.8Sr0.2Co0.8Ni0.2O3-8; and La0.7Sr0.3Fe0.8Ni0.2O0-8, and combinations thereof. A composite of such an electronically conductive, catalytically active material and an ionic conductor may be used. In certain embodiments, the ionic conductor comprises a material selected from the group consisting of yttria-stabilized zirconia, rare-earth-oxide-stabilized zirconia, scandia-stabilized zirconia, rare-earth doped ceria, alkaline-earth doped ceria, rare-earth oxide stabilized bismuth oxide, and various combinations of these compounds.
Typically, the composite electrolyte layer 38 is disposed between the cathode layer 34 and the anode layer 36. The main purpose of the electrolyte layer 38 is to conduct ions between the anode layer 36 and the cathode layer 34. The electrolyte carries ions produced at one electrode to the other electrode to balance the charge from the electron flow, and to complete the electrical circuit in the fuel cell. Additionally,, the electrolyte separates the fuel from the oxidant in the fuel cell. Typically, the composite electrolyte 38 is substantially electrically insulating. Accordingly, the composite electrolyte 38 is desirably stable in both the reducing and oxidizing environments, impermeable to the reacting gases, adequately ionically conductive at the operating conditions, and compliant with the adjacent anode 36 and cathode 34. The composite ceramic electrolyte described, for embodiments of the present invention has substantially high compliance, and superior gas-tight characteristics. These features provide distinct advantages over conventionally deposited ceramic electrolytes.
In some embodiments of the present invention, as discussed above, the composite ceramic electrolyte has a microstructure which comprises a first ceramic composition comprising a plurality of nano-dimensional microcracks and a second ceramic composition substantially embedded within at least a portion of the plurality of nano-dimensional microcracks. The first and the second compositions are different from each other. The composite ceramic electrolyte may have, any suitable first and second ceramic compositions, microcrack dimensions, and thicknesses, including those listed in the embodiments discussed previously. The composite ceramic electrolyte has a gas permeability, measured in air, of less than about 8×10−11 cm2Pa−1sec−1.
The anode, cathode, and electrolyte layers are illustrated as single layers for purposes of simplicity of explanation. It should be understood, however, that the anode layer may have a single/multiple layers in which the particle size is graded within the individual layer. The composition of the material may also be graded for thermal compatibility purposes. In another example, the electrolyte structure may be used for a tubular geometry. Furthermore, though the operation of the cell is explained with a simple schematic, embodiments of the present invention are not limited to this particular simple design. Various, other designs—some of them complex—are also applicable, as will be appreciated by those skilled in the art. For example, in certain embodiments, the fuel cell may comprise a composite electrode-electrolyte structure, rather than individual electrode (anode/cathode) and electrolyte layers. Such composite structures may also be incorporated with, electrocatalytic materials such as La1-xSrxMnO3 (LSM), La1-xSrxCoO3 (LSC), La1-xSrxFeO3 (LSF), SrFeCo0.5Ox, SrCo0.8Fe0.2O3-8; La0.8Sr0.2Co0.8Ni0.2O3-8; and La0.7Sr0.3Fe0.8Ni0.2O3-8, to enhance their performance. The fuel cell may comprise additional layers, such as buffer layers, support layers, and the like, helping to better match the coefficient of thermal expansion (CTE) of the layers. In addition, barrier layers may be included in the fuel cell to prevent detrimental chemical reactions from, occurring during operation. These layers may be in various forms, and may be prepared by various known techniques. As one example, the buffer/support layers may be a porous foam or tape, or in the form of a knitted wire structure.
Another embodiment of the invention is a method of making a composite ceramic electrolyte.
To start with, a first ceramic composition comprising a plurality of nano-dimensional microcracks is provided in step 62. The first ceramic composition layer may be fabricated, by any known process in the art, e.g., by thermal deposition techniques. Examples of suitable thermal deposition techniques include, but are not limited to, plasma spraying, flame spraying, and detonation coating. Such layers typically have nano-dimensional microcracks. Alternatively, the first ceramic composition layer may be deposited from a vapor phase such as plasma vapor deposition (PVD), electron beam plasma vapor deposition (EBPVD), or chemical vapor deposition (CVD). The ceramic layer may also be prepared by band casting or screen-printing a slurry, followed by subsequent sintering. Layers manufactured with such processes often contain capillary spaces, which are formed by pores and open microcrack structures.
In an exemplary embodiment, the first ceramic composition is deposited by an air plasma spray (APS) process. Plasma spray coatings are formed by heating a gas-propelled spray of a powdered metal oxide or a non-oxide material with a plasma spray torch. The spray is heated to a temperature at which the powder particles become molten. The spray of the molten particles is directed against a substrate surface, where they solidify upon impact to create the coating. The conventional as-deposited APS microstructure is typically characterized by a plurality of overlapping splats of material, wherein the inter-splat boundaries may be tightly joined, or may be separated by gaps resulting in some pores and microcracks. The ceramic electrolyte may be applied by an APS process, using equipment and processes known in the art. Those skilled in the art understand that the process parameters may be modified, depending on various factors, such as the composition of the electrolyte material, and the desired microstructure and thickness. Typically, the ceramic electrolyte comprising a plurality of nano-dimensional microcracks has a porosity less than about 10 volume percent. The as-deposited ceramic electrolyte is characterized by a gas permeability, measured in air, of less than about 8×10−10 cm2Pa−1sec−1.
A flow chart for an exemplary process 70 for forming a composite ceramic electrolyte is shown in
The first ceramic composition is infiltrated with a liquid precursor comprising at least one oxidizable metal ion. In certain embodiments, the liquid precursor is employed (or “used”) in the form of a solution. The solution may comprise any solvent and a soluble salt material that allows formation of the solution. The metals are present in the form of cations. The corresponding anions are inorganic compounds, for example nitrate NO3, or organic compounds, for example alcoholates or acetates. If alcoholates are used, then chelate ligands, such as acetyl acetonate, may be advantageously added to decrease the hydrolysis sensitivity of the alcoholates. Examples of suitable solvents are toluene, acetone, ethanol, isopropanol, ethylene glycol, and water. Aqueous and alcohol solutions of nitrates, and organic-metallic soluble materials, such as oxalates, acetates, and citrates, may also be used. The solution desirably has suitable wettability and solubility properties to permit infiltration into the pores and microcracks. Infiltration and heating of the first ceramic composition with the second ceramic composition typically lead to decrease in porosity. In one embodiment, the porosity reduction is from about 8% of the volume to about 5.8% of the volume, an approximate decrease in crack volume of about 25%.
When the electrolyte comprises an oxide of a metal “Me”, where “Me” is Zr, Ce, Y, Al or Ca, the precursor solution may comprise a nitrate Me(NO3)x, where x=2 for Ca, and x=3 for Zr, Ce, Y, Al, Co, Mn, Mg, Ca, Sr, Y, Zr, Al, Ti. Alternatively (or in addition), the precursor solution may comprise a lanthanide, such as Ce, Eu or Gd. The metal nitrates are generally available as crystalline hydrates, for example Ce(NO3)3.6H2O, which are easily soluble in water. Metal nitrates decompose into the corresponding oxides at elevated temperatures, while simultaneously forming gaseous NO2. The conversion temperature at which oxide formation results is known for many of the nitrates and, accordingly, the processing conditions are chosen.
Typically, the oxidizable metal ion may be thermally converted into a metal oxide. After infiltrating a desired number of microcracks, the solvent is evaporated as the temperature increases under heat input, and the metal changes into the metal oxide at an elevated temperature, thereby closing the infiltrated microcracks. As used herein, “closing a selected number of nano-dimensional microcracks” encompasses reducing the dimension of the nano-dimensional microcracks by filling the nano-dimensional microcracks, or by closing the surfaces of the cracks. In the heat treatment, the heat input can be carried out by various techniques, e.g., in a thermal oven, in a microwave oven, with a heat radiator, or with a flame. A multiple repetition of the infiltration and healing processes may be carried out in order to achieve any specific microstructure and gas permeability values.
The embodiments of the present invention are fundamentally different from those conventionally known in the art. There have been reports of infiltrating highly porous ceramic layers with metal ions, and heat treating them in order to density the ceramic layer. In such cases, the initial ceramic layers are highly porous (porosity>10%) and have micron-sized microcracks that result in relatively higher gas permeability (higher than 3.5×10−10 cm2Pa−1sec−1 measured in air) after infiltrating with metal ions. As a result, such processed products have different characteristics, compared to the composite electrolytes described heroin.
The following examples serve to illustrate the features and advantages offered by the present invention, and are not intended to limit the invention thereto.
Example. Preparation of composite yttria-stabilized zirconia (YSZ)-gadolinium doped ceria (GDC).
Gadolinium and cerium nitrate aqueous precursor solutions were prepared and mixed in the appropriate ratios to yield a 1.2 M solution with a 20 mol % Gd doped CeO2 (20GDC) final composition, after nitrate decomposition and oxidation. A one inch (2.54 cm) diameter porous stainless steel substrate with a 65 micron thick 8 mol % yttria stabilized zirconia (8YSZ) air plasma sprayed (APS) electrolyte was used as a baseline. The 20GDC nitrate solution was painted at 3.5 mg/cm2 onto the APS coating, during which the solution visibly wicked into the permeable coating. The substrate was air dried at room temperature and 70° C. for approximately 5 minutes each. The substrate was then placed in a furnace at 300° C. for 1.5 minutes, and then allowed to cool at room temperature. Once fully cooled, the process of painting 20GDC and heat treating at 300° C. was repeated, until a total of 4 treatments were made. A fifth 20GDC painting was applied, after which the sample was heat treated to 500° C. for 0.5 hrs. The four −300° C. heat treatments and the 500° C. process was iterated twice.
A micrograph of a typical as-deposited APS electrolyte structure is shown in
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof, without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention, without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
