Patent Application
20070092395
This invention relates to the storage and production of hydrogen. More particularly, this invention relates to materials used to store and generate hydrogen in such systems, and methods for making such materials.
Hydrogen is a “clean fuel” because it can be reacted with oxygen in hydrogen-consuming devices, such as a fuel cell or a combustion engine, to produce energy and water. Virtually no other reaction byproducts are produced in the exhaust. As a result, the use of hydrogen as a fuel effectively solves many environmental problems associated with the use of fossil fuels. Safe and efficient storage and retrieval of hydrogen gas is, however, essential for many applications that can use hydrogen. In particular, minimizing volume and weight of the hydrogen storage systems are important factors in mobile applications.
Several methods of storing hydrogen are currently used but these are either inadequate or impractical for widespread mobile consumer applications. For example, hydrogen can be stored in liquid form at very low temperatures, but liquefying the gas consumes significant amounts of energy, and evaporation of the hydrogen poses a considerable problem. Moreover, storing compressed gaseous hydrogen presents problems of capacity. Ordinary compressed gas tanks hold only about 1% of hydrogen gas by weight. More expensive composite cylinders with special compressors can store hydrogen at higher pressures to achieve a more favorable storage ratio of about 10% by weight. Although even higher pressures are possible, safety factors and the high amount of energy consumed in achieving such high pressures have compelled a search for alternative hydrogen storage technologies that are both safe and efficient.
Several different metal hydrides have been extensively studied as potential solid-state storage media for hydrogen fuel systems. For example, hydrides of aluminum with alkali metals such as sodium, potassium, and lithium have been shown to release hydrogen at temperatures that often fall within a practical range for mobile consumer applications (e.g., below about 200° C.). However, these materials tend to behave irreversibly within this practical temperature range; that is, they release hydrogen in considerable amounts, but do not absorb (store) hydrogen with practical efficiency at these temperatures. Therefore, there is a need to provide materials having a high gravimetric hydrogen storage capacity and the ability both to store and produce hydrogen at practical temperatures. There is also a need for methods for the production of such materials.
Embodiments of the present invention meet these and other needs. One embodiment is a method for making a material. The method comprises providing a metallic catalyst comprising at least one metal component and at least one Group 13 element, wherein the catalyst comprises a disordered phase; providing a matrix material; and homogenizing the catalyst with the matrix material to form a material comprising (i) the matrix material and (ii) the catalyst disposed within the matrix material.
Another embodiment is a hydrogen storage material. The material comprises a matrix material and a metallic catalyst disposed within the matrix material, the catalyst comprising at least one metal component and at least one Group 13 element, wherein the catalyst comprises a disordered phase. The hydrogen storage material has a hydrogen desorption rate of at least about 3 weight percent per hour at a temperature of about 150° C.
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 drawings, wherein:
Complex hydrides, such as, for example, hydrides containing aluminum and one or more alkali metals, have been shown to reversibly store (absorb) and release (desorb) at moderate temperatures (that is, temperatures below about 200° C.) when they are doped with certain catalyst compounds. For example, in Bogdanovic et al., U.S. Pat. No. 6,106,801, transition metal compounds have been reported to provide this catalyst effect, albeit with slow and unstable kinetics. A number of processes for the synthesis of doped complex hydrides have been proposed, but many of these processes involve additions of salts to a hydride starting material. The salt additions ultimately detract from the overall storage capacity of the final material by consuming at least a portion of the starting material, rendering the consumed material unavailable for hydrogen storage.
A recent process reported by Gross et al. in U.S. 2003/0165423 eliminates the use of salts as vehicles for introduction of transition metals into complex hydrides. In the method of Gross et al., titanium aluminide (TiAl3) powder is mixed with aluminum and a simple alkali hydride, such as sodium hydride, to form a titanium-doped complex alkali metal/aluminum hydride. The titanium aluminide used in the Gross et al. method is described to be a product of the reaction of titanium chloride with aluminum metal and sodium hydride, and is further described to have the L12 crystal structure. This ordered structure can be envisioned as a cube where the aluminum atoms are positioned in the center of each face of the cube and the titanium atoms are positioned at the corners of the cube. In an example described by Gross et al., powders of titanium aluminide, aluminum, and sodium alanate (NaAlH4) were mixed in a high-energy ball mill for 2 hours. The resultant material was then hydrogenated by exposure to hydrogen gas at elevated temperature and pressure. Although the material doped with titanium via this method showed an increase of over a factor of ten in hydrogenation kinetics over undoped material, the data reported in U.S. 2003/0165423 suggest that the rate of absorption obtained with the Gross et al. material may still be too low for many practical applications.
The present inventors have discovered a source of the problems causing the above shortcomings. According to this discovery, the capacity and kinetics of some complex hydrides can be significantly improved over the results noted in the prior art by doping with a catalyst component having a composition and crystal structure that closely matches that of the catalyst as it is found to exist in the doped hydride material. Without being limited to any particular explanation, it is theorized that providing a catalyst component very close in composition and structure to the catalyst found to form in the doped hydride minimizes the need for intermediate reactions and/or phase transformations during processing, thereby allowing for more efficient doping of the hydride with the catalyst.
The present inventors have investigated certain complex hydrides and their behavior upon doping, and after complex mechanistic studies they have found that the dopants exist as a discrete, disordered phase in a mixture with the hydrides. Disordered phases have no long-range order in the positioning of atoms within the crystal lattice; unlike the L12 structure noted above, the atoms in disordered titanium aluminide are not arranged in accordance with any particular pattern. Thus, according to the discovery of the present inventors, the addition of a catalyst having a disordered structure will allow for improved performance of the resultant hydrogen storage material when compared to the performance of material formed by addition of a catalyst having an ordered structure.
Accordingly, one embodiment of the present invention is a method for making a material. The method comprises providing a metallic catalyst comprising at least one metal component and at least one Group 13 element. A matrix material is also provided, and the catalyst is homogenized with the matrix material. The resultant hydrogen storage material comprises the catalyst disposed within a matrix material.
The provided catalyst comprises a disordered phase, such as, for example, a disordered face-centered cubic phase. The existence of the disordered phase in substantial amounts in the catalyst increases the advantageousness of the catalyst. In some embodiments, the catalyst comprises at least about 60% by volume of the disordered phase. In certain embodiments, the catalyst comprises at least 75% by volume, and in particular embodiments at least 90% by volume, of the disordered phase. In a specific embodiment, the catalyst consists essentially of the disordered phase.
As noted above, transition metal elements appear to be effective dopants for certain complex hydride systems. In some embodiments, the metal component of the metallic catalyst comprises at least one transition metal element. Suitable examples of transition metal elements include, but are not limited to titanium, vanadium, iron, niobium, zirconium, yttrium, scandium, and combinations of these.
The catalyst also comprises at least one Group 13 element (i.e., boron, aluminum, gallium, indium, or thallium). Aluminum is an advantageous element in that it is quite common and is the lightest of the Group 13 elements. Therefore, in particular embodiments, the at least one Group 13 element of the metallic catalyst comprises aluminum. The molar ratio of the Group 13 element to the metallic component, in certain embodiments, is in the range from about 0.01:1 to about 100:1. In particular embodiments, this range is from about 0.01 to about 3:1. In a specific embodiment, this range is about 3:1.
Several of the more commonly known complex hydrides, including the alanates, contain aluminum. Moreover, the work of the present inventors and others indicates that compounds of transition metals, particularly titanium, with aluminum are effective catalysts for use in alanate systems. Accordingly, in certain embodiments the at least one metal component comprises titanium and the at least one Group 13 metal comprises aluminum. In particular embodiments the metallic catalyst comprises a disordered phase of titanium aluminide. This disordered phase, in certain embodiments, is face-centered cubic. However, it will be appreciated that “disordered phase of titanium aluminide” as used herein refers to an atomically disordered phase of any compound of titanium and aluminum and is not limited to one particular compound. For example, depending on the relative amounts of titanium and aluminum present, the disordered titanium aluminide will have any of a range of possible compositions, including TiAl3, TiAl, Ti3Al, or even non-stoichiometric formulations.
In many cases, a catalyst comprising a disordered phase is not readily available and therefore must be synthesized prior to homogenizing with the matrix material. In these instances, providing the metallic catalyst comprises providing a primary metal and transforming the primary metal into the metallic catalyst. Providing the primary metal comprises, in some embodiments, providing a material comprising a transition metal element (such as titanium), a Group 13 element (such as aluminum), or combinations of both (including compounds of both, such as, for example, titanium aluminide). Often the primary metal is provided as a combination or compound of materials in a predetermined molar ratio. For instance, in some embodiments the primary metal comprises aluminum and titanium in a predetermined molar ratio. The particular ratio selected is determined based upon the desired phase to be formed during the transforming step. In some embodiments, the molar ratio is in the range from about 0.01:1 to about 100:1, in terms of moles aluminum to moles titanium. In certain embodiments, this range is from about 0.01 to about 3:1. In a specific embodiment, this range is about 3:1, such as, for instance, where the primary metal comprises TiAl3.
The primary metal is transformed into the metallic catalyst. In certain embodiments, transforming comprises inducing a phase transformation within the primary metal to form the disordered phase of the catalyst. As used herein, “phase transformation” includes any change to the crystal structure of the material, even where no change in chemical composition occurs. An example of such a transformation is a change in crystal structure in TiAl3 from an ordered crystal structure to a disordered face-centered cubic structure. Inducing such a transformation, in some embodiments, is achieved by subjecting the primary metal to stress, strain, heat, or any combination of these. Ball milling, grinding, shot peening, mixing, crushing, swaging, extruding, and cold working are all suitable methods for inducing a phase transformation in accordance with embodiments of the present invention. Quenching the primary metal from a high temperature to a low temperature can also be used in certain situations to elicit the required transformation. Thermal spraying, rapid solidification, and atomization are suitable techniques because the rapid cooling of the material is often sufficient to induce phase transformations. Moreover, condensing material from a gas or plasma, as in, for example, sputtering, is another potentially suitable technique for transforming the primary metal into the metallic catalyst. The particular technique chosen will depend upon several factors, such as, for example, the composition of the primary metal, the quantities being processed, and the desired metallic catalyst to be synthesized. In particular embodiments, transforming comprises ball milling the primary metal for at least 2 hours, such as via a high-energy ball milling apparatus used commonly in the art. The transforming step is continued until the desired amount of primary metal has been transformed. In some cases substantially all of the primary metal is transformed, while in alternative embodiments, only a portion of the primary metal is transformed. In these latter alternatives, then, the metallic catalyst will comprise some disordered material and some ordered material. In the former embodiments, the metallic catalyst comprises substantially 100% disordered material.
In certain embodiments of the present invention, the surface area of provided catalyst is an important factor in ensuring efficient incorporation of the catalyst into the end material. Thus, where the catalyst is provided as a powder, the size of the powder particles is often a consideration, because a comparatively small particle size generally creates a comparatively large surface area to volume ratio. In some embodiments, providing the catalyst comprises providing a powder having a mean particle size of up to about 1 micrometer. In certain embodiments, the mean particle size is less than about 100 nanometers. In particular embodiments, the mean particle size is less than about 20 nanometers.
A matrix material is provided for homogenization with the metallic catalyst. In certain embodiments the matrix material comprises a hydrogen storage material, such as a complex hydride, that will benefit from doping with the metallic catalyst. In alternative embodiments, the matrix material comprises multiple materials. The matrix material, in certain embodiments, comprises a hydride, a metal, a semi-metal, or a combination of these.
In some embodiments, the hydride of the matrix material comprises a material having the general chemical formula An(MHz)x, wherein
A is at least one element selected from the group consisting of elements from Groups 1, 2, 3, 4, and 12 of the Periodic Table;
M is at least one element selected from the group consisting of the Group 13 elements;
n is a number in the range from about 1 to about 3;
z is a number in the range from about 4 to about 6; and
x a number in the range from about 1 to about 6.
In certain embodiments, M comprises aluminum. In particular embodiments, the hydride comprises an alanate compound, and thus n is about 1 and z is about 4. A particular example of such a compound is sodium alanate, NaAlH4, mentioned previously, although it will be clear from the above description that embodiments of the present invention contemplate the application of this method to a wide variety of compounds. In some embodiments, n is about 3 and z is about 6. A particular example of such a compound, though again not limiting, is Na3AlH6.
In certain embodiments, the matrix material comprises a combination of a hydride and aluminum. In many of these embodiments, the aluminum serves the function of a reactant during the hydrogen storage process, reacting with the hydride in the presence of hydrogen and the catalyst to form a different hydride having desirable storage and/or desorption properties . For example, in some embodiments, the hydride comprises Na3AlH6. In some embodiments, moreover, the hydride comprises sodium hydride, NaH. Such hydrides, in the presence of the catalyst, can react with the aluminum and hydrogen to form sodium alanate, for example.
The metal of the matrix material, in some embodiments, comprises at least one of the first-row transition metal elements (i.e., scandium through zinc), at least one of the Group 13 elements, or combinations of any of these. The matrix material, in some embodiments, comprises a mixture of sodium hydride and aluminum. As described above, this particular mixture can be reacted to form sodium alanate.
The catalyst and matrix materials are provided and are homogenized. In many cases the relative amounts of these species present during homogenization is a factor in determining the properties of the final material. In some embodiments, the catalyst is provided in an amount up to about 10 mole percent relative to the amount of matrix material provided. In certain embodiments, the amount of catalyst is in the range from about 0.1 to about 2 mole percent relative to the amount of matrix material. Homogenization may be accomplished by any of the methods described previously for transforming primary metal into metallic catalyst. In particular embodiments, homogenizing comprises ball milling for at least about 15 minutes. The actual time, temperature, and other selected conditions for homogenizing will depend on the characteristics of the materials being homogenized.
In order to fully capitalize on the advantages provided by the method described above, one embodiment of the present invention is a method for making a material. The method comprises providing a primary metal comprising at least one material selected from the group consisting of titanium, aluminum, TiAl3, and combinations of any of the foregoing; ball-milling the primary metal to form a metallic catalyst, wherein the metallic catalyst consists essentially of a disordered face-centered cubic phase of titanium aluminide; providing a matrix material comprising at least one material selected from the group consisting of a hydride, aluminum, and combinations thereof; and ball milling the matrix material with the catalyst.
Hydrogen storage materials synthesized by the methods described above can be hydrogenated in accordance with any suitable method to produce a hydrogen storage material. For instance, the homogenized material may be removed to a reactor vessel capable of exposing the material to elevated hydrogen pressure. Moreover, the reactor may be capable of temperature control to allow exposure of the material to hydrogen at elevated temperature and pressure. As a non-limiting example, for alanates and other complex hydrides, suitable hydrogenation can be achieved by exposing the material to hydrogen pressures of above 50 atm at temperatures above 100° C. for times ranging from about a minute to a few days, depending on the desired level of hydrogenation and the rate at which the hydrogenation reaction takes place under the given conditions. On the other hand, hydrogen release (desorption) is simply achieved by heating hydrogenated material in hydrogen-lean environments. The temperature selected for desorption will depend on the material being heated and the desired rate of hydrogen release.
The methods provided in accordance with embodiments of the present invention produce materials having markedly and unexpectedly improved hydrogen storage and release characteristics than similar materials made using prior art methods. Consequently, embodiments of the present invention also include thehydrogen storage materials made by the methods described above.
In accordance with embodiments of the present invention, a hydrogen storage material comprises the matrix material and the metallic catalyst disposed within the matrix material. The compositional alternatives for the matrix material and the catalyst are as described above for the method embodiments. In stark contrast to conventional hydrogen storage materials of similar chemical composition, the hydrogen storage material of the present invention has a hydrogen desorption rate of at least about 3 weight percent per hour at a temperature of about 150° C. This rate is significantly higher than that reported for other hydrogen storage materials of similar type, and better enables the use of such materials in practical systems for fuel storage and retrieval.
In order to more fully illustrate and describe the methods and materials disclosed herein, the following examples are provided. However, it should be appreciated that these examples are not intended to limit the invention in any way.
A 5 gram quantity of TiAl3, having a D022 ordered crystal structure as received from the manufacturer, was milled in a ball mill for 8 hours to yield the disordered TiAl3 alloy. The disordered alloy was then milled for 30 minutes with NaH and Al in a 0.02 TiAl3:1 NaH:1Al molar ratio, resulting in a hydrogen storage material comprising the disordered titanium aluminide disposed in a matrix material made of sodium hydride and aluminum This material was then hydrogenated at 120° C. C under 130 bars of hydrogen gas pressure.
Desorption rate of hydrogen from the hydrogenated material described above as a function of temperature was measured by observing the desorption of hydrogen under isothermal conditions into a known volume.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations, equivalents, or improvements therein may be made by those skilled in the art, and are still within the scope of the invention as defined in the appended claims.
