Patent Application
20080032183
The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings in which:
The electrolyte matrix comprises a support material coated with alkaline precursors. The support material comprises a porous ceramic material having a sub-micron particle size. In this illustrative example, LiAlO2, including γ-LiAlO2, α-LiAlO2 and β-LiAlO2, are used as the support material.
The alkaline precursor material comprises an alkaline containing compound which has a low melting point and/or is soluble in water or in a predetermined solvent. Suitable alkaline containing precursors include alkaline hydroxides, alkaline isopropoxides, alkaline nitrates, alkaline acetates, alkaline oxalates and mixtures thereof. In particular, lithium acetate, lithium acetate dehydrate, lithium oxalate, lithium nitrate and lithium hydroxide are suitable for use as alkaline precursor materials.
In certain embodiments, the electrolyte matrix also comprises one or more additive components which may include binder, plasticizer and other suitable materials. It is also understood that other materials may be suitable for use in the electrolyte matrix 2 of the fuel cell 1.
As above-indicated, the electrolyte matrix 2 of
As shown in
In this illustrative embodiment, the support material and the alkaline precursor material are provided in a powder form, such that the support material has a particle size of about 1 micron and the alkaline precursor material has a particle size of 50 microns or less. It is understood that the support material and the alkaline precursor material may be pre-milled to achieve the desired particle size.
In a third step S103, the support material provided in step S101 and the alkaline precursor material provided in step S102 are dry mixed for a first predetermined period of time to provide a relatively uniform mixture. Conventional methods, such as dry blending the materials in a blender, may be employed in the third step S103. In certain illustrative embodiments, the first predetermined period of time is about 30 minutes.
In the next steps the mixture of support material and alkaline precursor material is processed so that the alkaline precursor material coats the support material to form coated support material. In particular, in a fourth step S104, the mixture formed in the third step S103 is heated to a predetermined temperature for a second predetermined time period. The predetermined temperature is between 60° C. and 400° C. so as to melt the alkaline precursor material in the mixture without vaporizing or significantly decomposing the precursor material. The second predetermined time period is a time period sufficient to completely melt the alkaline precursor material and to coat the particles of the support material completely. In certain embodiments, the mixture may be heated again to a temperature higher than the predetermined temperature for a third predetermined time period so as to ensure that the molten alkaline precursor material completely coats the particles of the support material.
The heated mixture is thereafter allowed to cool in a fifth step S105 so as to solidify the alkaline precursor material coating the support material particles and to produce coated support material. The coated support material in this illustrative embodiment is allowed to cool to about room temperature. Although not shown in
In step S106, cooled coated support material is comminuted and sieved to break apart any granules formed during the heating and cooling processes in steps S105 and S106 and to provide a relatively homogeneous powder. Comminution in step S106 may be achieved by dry milling the coated support material in a grinding jar or by using any other process known in the art. Sieving of the coated support material powder is performed in order to remove any large granules remaining after the comminution process is performed. In this way, the coated support material powder having a relatively uniform particle size and suitable for use in manufacturing the electrolyte matrix, is produced. As shown in
As discussed herein above, the alkaline precursor material may be an alkaline material which is soluble in water or in another pre-selected solvent.
As shown in
As mentioned above, the alkaline precursor material is soluble in water or in another suitable solvent and has a particle size of about 50 microns or less. Suitable alkaline precursor materials include lithium oxalate and lithium hydroxide, both of which are soluble in water.
As discussed above, the second predetermined amount of the alkaline precursor material is relative to the first predetermined amount of the support material. In this illustrative embodiment, the second predetermined amount of alkaline precursor material, provided in step S202, is between 5 and 100 volume % of the first predetermined volume amount of the support material.
In the next step S203, the support material provided in step S201 and the alkaline precursor material provided in step S202 are mixed in a predetermined amount of solvent. The mixing in step S203 may be accomplished by blending the support material, the alkaline precursor material and the solvent using a blender for a predetermined time period or until the precursor material completely dissolves in the solvent. It is understood that any other suitable state-of-the-art mixing processes may be employed in step S203. The amount of solvent used in the mixture should be sufficient to completely dissolve the alkaline precursor material.
The mixture is then processed to cause the alkaline precursor material to coat the support material. Specifically, the solution produced in step S203 is then dried in step S204 to remove the solvent from the solution and to provide support material particles coated with the precursor material. Spray drying and/or heating, or another process known in the art, may be used to dry the solution in step S204. The resulting coated support material may also be heated in step S205 to a predetermined temperature for a predetermined time period in order to remove any remaining solvent from the mixture and to promote the coating of the support material with the alkaline precursor material. The heated coated support material is thereafter cooled in step S206.
In the next step S207, the coated support material produced in step S206 (or in step S204, if no heating is used) is comminuted and sieved so as to break apart or remove any large granules and to provide a substantially homogeneous powder comprising coated support material particles. This step S207 is similar to step S106 described herein above with respect to
The coated support material prepared in accordance with methods shown in
As shown in
In a third step S303 of the matrix fabrication method, the coated support material provided in step S301 and the dispersant provided in step S302 are mixed so as to form a slurry mixture. In this step, the mixture of coated support material and the dispersant may be milled for a predetermined period of time to break down any agglomerates present and to ensure that the coated support material particles are uniformly dispersed throughout the slurry. The milling is accomplished using any state-of-the-art milling technique, such as ball milling, attrition milling or fluid energy grinding. For example, the slurry mixture may be milled using the ball milling technique using YTZ grinding media. The size of the grinding media is based on the desired particle size of the coated support material particles in the slurry.
In the next step S304, one or more additives are added to the slurry mixture. For example, aluminum powder may be added to the slurry mixture in step S304 as an additive for strengthening the electrolyte matrix. After the addition of the additives in S304, the resulting mixture is again mixed or milled in step S305. As in step S303, state-of-the-art mixing or milling techniques, such as ball milling, attrition milling or fluid energy milling may be used in step S303. The mixture is mixed/milled in S305 for a sufficient period until the additives become uniformly dispersed throughout the slurry mixture and any agglomerates present in the mixture are broken down.
Following the mixing/milling in step S305, the slurry is formed into one or more electrolyte matrix elements in step S306 of the method. The electrolyte matrix elements may be formed using any suitable state-of-the-art technique. In the illustrative example shown in
The completed matrix element formed using the method of
The optimal components and amounts of those components used to fabricate the coated support material and the components used in manufacturing the matrix using the above-described methods are dependent on the particular application and requirements of the fuel cell. Illustrative examples of fabricating the coated support material and manufacturing the electrolyte matrix are described herein below.
In this illustrative example, α-LiAlO2 powder is used as the support material in the matrix and lithium acetate powder is used as the alkaline coating material. The method shown in
In the first step S101, the support material α-LiAlO2 is provided in powder form having a particle size of about 0.1 micron and a surface area of about 10 m2/g. The predetermined amount of the α-LiAlO2 material provided is about 85% of the total volume of the mixture. In the second step S102, the low melting point alkaline material lithium acetate de-hydrates with a melting point of about 58° C. is provided in powder form. Lithium acetate has a particle size of about 50 microns. The predetermined amount of lithium acetate provided is about 15% of the total volume of the mixture.
The α-LiAlO2 support material and the lithium acetate material are dry mixed in the third step S103 by blending the mixture in a blender for about 30 minutes. The blended mixture of α-LiAlO 2 and lithium acetate prepared in the third step S103 is heated in step S104 to about 65° Celsius for 3 hours so as to melt the lithium acetate to coat the α-LiAlO2 support material. The mixture is thereafter heated to 180° C. for an additional 3 hour time period so as to drive off any water present in the mixture. The heated mixture formed in step S104 is then cooled to room temperature in step S105 to form coated α-LiAlO2 support material. The coated α-LiAlO2 support material is then examined using state-of-the-art SEM and BET techniques to determine the surface area and particle size of the coated α-LiAlO2.
In the next step S106, the coated α-LiAlO2 support material is comminuted using conventional dry milling in a grinding jar for a time period of 24 hours so as to grind any granules and produce a substantially homogeneous coated α-LiAlO2 powder. The resulting coated α-LiAlO2 powder then undergoes a conventional sieving operation so as to remove any large granules present in the powder which have not been ground by the dry milling operation.
The sieved coated α-LiAlO2 support material is then used to form a slurry mix and to fabricate the matrix from the slurry mix. The method shown in
The mixture of the coated α-LiAlO2 and the dispersant is then milled in step S303 using a conventional ball milling technique to produce a slurry. The grinding media suitable for ball milling the mixture of the coated α-LiAlO2 and the dispersant is YTZ grinding media having a 6 mm diameter. The mixture is milled for 24 hours, or until the coated α-LiAlO2 is sufficiently dispersed in the dispersant. In the fourth step S304, aluminum powder is added as an additive to the slurry mixture. The amount of the aluminum powder additive is 9 wt % of the solids, and the particle size of the aluminum powder is preferably about 1-5 micron. The mixture of coated α-LiAlO2 material, dispersant and aluminum powder is then milled in step S305 for a period of about 18 hours using ball milling with 6 mm YTZ grinding media. The resulting slurry mixture can then be used to fabricate matrix elements.
In this illustrative example, the matrix elements are fabricated from the slurry mixture prepared in step S305 using a tape casting technique. In particular, the slurry is tape cast using a doctor blade in step S306 and dried in step S307. The resulting matrix element is a flat and flexible green tape suitable for use in the fuel cell. It is understood that the size and the dimensions of the matrix element fabricated using this method will vary depending on the fuel cell requirements.
In this illustrative example, α-LiAlO2 powder is provided for use as the support material in the matrix and lithium oxalate powder is provided as the alkaline coating material. The method shown in
In the first step S101 of preparing the coated support material, the support material α-LiAlO2 is provided in powder form. The α-LiAlO2 support material has a particle size of about 0.1 micron and a surface area of about 10 m2/g. The predetermined amount of α-LiAlO2 support material provided is about 75% of the total volume of the mixture. In the second step S102, the alkaline material lithium nitrate is provided in powder form. The lithium nitrate alkaline material has a particle size of about 50 micron and a surface are of about 10 m2/g. The predetermined amount of lithium nitrate material provided in this step is about 25% of the total volume of the mixture. It is understood that the total volume of the mixture depends on the number and size of the matrix elements to be manufactured using the coated support material.
In the third step S103, the α-LiAlO2 support material and the lithium nitrate alkaline material are dry mixed, or dry blended, using a blender for about 30 minutes. The blended mixture is thereafter heated in step S104 to about 300° C. for a time period of about 3 hours in order to melt the lithium nitrate to coat the α-LiAlO2 support material. The temperature of the mixture is then increased to about 400° C. at a rate of 5° C./min and the heating of the mixture is continued for an additional time period of about 1 hour at about 400° C. to complete the coating of the support material and to drive off any water present in the mixture. The heated mixture formed in step S104 is allowed to cool to room temperature in the fifth step S105, forming coated α-LiAlO2 material. The resulting coated α-LiAlO2 may be examined using state-of-the-art SEM and BET techniques to determine its surface area and particle size.
In step S106, the coated α-LiAlO2 material is comminuted using the conventional dry milling technique. In particular, the coated α-LiAlO2 material is dry milled in a grinding jar for 24 hours to grind away any granules and to form a substantially homogeneous coated α-LiAlO2 powder. The milled coated α-LiAlO2 powder is then sieved to remove any remaining large granules present in the powder.
The milled and sieved coated α-LiAlO2 material produced in step S106 is then used in matrix fabrication using the method shown in
The mixture of coated α-LiAlO2 and the dispersant formed in step S302 is then milled in step S303 using a conventional ball milling technique to produce a slurry. Ball milling is performed using YTZ grinding media having a 6 mm diameter for a period of about 24 hours, or until the coated α-LiAlO2 is sufficiently dispersed in the dispersant. In the next step S304, aluminum powder is added as an additive to the slurry mixture. The amount of the aluminum powder used is about 9 wt % of the solids, and the particle size of the aluminum powder is about 1-5 micron. The mixture of the coated α-LiAlO2, dispersant and aluminum powder is thereafter milled in step S305 for a period of 18 hours using the ball milling technique with the 6 mm YTZ grinding media. The resulting slurry mixture can be utilized in fabricating the matrix elements.
In this example, the matrix elements are formed from the slurry mixture formed in step S305 using the conventional tape casting technique. In particular, the slurry mixture is tape cast using a doctor blade in step S306 and dried in step S307 to form a flat and flexible green tape. As in the previous example, the dimensions of the matrix element fabricated using the method described above may vary depending on the requirements of the fuel cell system.
In this illustrative example, α-LiAlO2 powder is used as the support material in the matrix and lithium acetate powder is used as the alkaline coating material. The method shown in
In the first step S101 of preparing the coated support material, the support material α-LiAlO2 is provided in powder form having a particle size of about 0.09 micron and a surface area of about 20.7 m2/g. The predetermined amount of α-LiAlO2 provided in this step is about 50% of the total volume of the mixture. In the second step S102, water-soluble alkaline material lithium acetate is provided also in powder form. The water-soluble lithium acetate material used in this example preferably has a particle size of less than 50 microns and is provided in an amount of about 50% of the total volume of the mixture.
In the third step S103, the α-LiAlO2 support material and the lithium acetate material are mixed in the presence of water as the solvent in a blender for about 120 minutes. After the mixing in step S103 is completed, the mixture is dried in step S104. In particular, the mixture of α-LiAlO2 and lithium acetate dissolved in water is poured into a flat aluminum tray and heated to about 120° C. for about 24 hours to dry off the water present in the mixture. In the next step S105, the mixture is heated to about 400° C. at a rate of about 5° C./min and then heated at 400° C. for a time period of about 1 hour under an air flow so as to remove any water remaining in the mixture and to coat the α-LiAlO2 particles with the aluminum acetate material. The dried mixture is then allowed to cool in step S106 to room temperature, resulting in a coated α-LiAlO2 support material coated with lithium acetate. The coated α-LiAlO2 may be examined using SEM and BET techniques to determine the surface area and particle size of the coated powder. In the next step S107, the coated α-LiAlO2 is comminuted using the ball milling technique for about 24 hours to produce a substantially homogeneous coated α-LiAlO2 support material. In particular, YTZ grinding media having 6 mm diameter is used for ball milling the coated α-LiAlO2 powder. The resulting coated α-LiAlO2 is then sieved in order to remove any large granules remaining in the α-LiAlO2 powder.
The sieved coated α-LiAlO2 support material can then be used to form a slurry mixture and to fabricate one or more matrix elements. The method shown in
The electrolyte matrix elements fabricated in accordance with the above described methods and examples had an improved pore structure and experienced no significant change in pore size after being used in the fuel cells. In particular, the electrolyte matrix elements produced using the above methods had a smaller mean pore size and a narrower pore size distribution as compared with conventional electrolyte matrix elements. Such improved pore structure results in improved mechanical strength and endurance of the matrix when used in the fuel cell and in greater electrolyte retention by the matrix. Moreover, the matrix elements produced in accordance with the above methods experienced significantly smaller pore growth after being used in the fuel cell. This results in improved electrolyte retention by the matrix during the operation and over the life of the fuel cell.
In
The matrix elements formed from the coated α-LiAlO2 also showed no significant changes in porosity after being used in the fuel cell system. Pore size distribution of electrolyte matrix elements fabricated from coated α-LiAlO2 and of conventional matrix elements was measured before using the matrix elements in cell tests. The matrix elements were thereafter used in fuel cell tests at an operating temperature of 650° C. for 100 hours, after which the pore size distribution of these matrix elements was measured. The pore size distribution of the matrix elements before use in cell tests was then compared with the pore size distribution of the matrix elements after being used in cell tests.
As also shown in
In contrast, matrix elements fabricated from the coated α-LiAlO2 material experienced little or no pore growth after being used in the fuel cell operating for 100 hours at 650° C., such that the pore size distribution in these matrix elements remained substantially the same. This improvement in the relatively constant pore size distribution in the matrix elements formed from the coated α-LiAlO2 materials results in improved mechanical integrity and electrolyte retention of the matrix, as well as increased operating life of the fuel cell and improved fuel cell performance over the operating life of the fuel cell.
As shown in
Similarly, as shown in
As can be seen from these results, the improved electrolyte retention by the matrices fabricated in accord with the invention results in a significant increase in the operating life of the fuel cells, nearly doubling the operating life of the cells. The operating life of the fuel cells is also extended by the improvement in the matrix strength and reduced risks of cracking.
In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and the scope of the invention.
