DETAILED DESCRIPTION

[0031] FIGS. 2A and 2B illustrate an embodiment for storing hydrogen in an sp² bonded nanostructured storage material 10 with a triangular lattice that has a flat planar network.

[0032] FIG. 2A shows the plan view of nanostructured storage material 10 with a triangular lattice structure, without the hydrogen molecules. The chemical composition of nanostructured storage material 10 is of the AB type, an example of which is boron nitride. Light elements 16 and light elements 20 of nanostructured storage material 10 are shown with bigger and smaller empty circles, respectively. Light elements 16 and 20 are coupled with sp² bonds 24. For a discussion of suitable light elements see below.

[0033] FIG. 2B shows a perspective view of the triangular lattice, wherein hydrogen molecules 28 are shown in their bonding position. In some embodiments hydrogen molecules 28 are positioned over the center of the hexagons of the triangular lattice, in other embodiments hydrogen molecules 28 are positioned over a bond or over an atom of the lattice. In some embodiments there is one energetically favored position for hydrogen molecules 28, in other embodiments there are several substantially equivalent positions. In some embodiments the orientation of hydrogen molecules 28 can be parallel to the local plane of the triangular lattice, in other embodiments the orientation can be perpendicular, or it can make some other angle with the local plane of the triangular lattice.

[0034] The adsorption properties of hydrogen to nanostructured storage material 10 depend on the nature of sp² bonds 24. In some embodiments this property is used, when the adsorption properties of hydrogen to nanostructured storage material 10 are improved by modifying sp² bonds 24.

[0035] Embodiments with modified sp² bonds 24 are described, for example, in U.S. patent application entitled: “Increasing Hydrogen Adsorption For Hydrogen Storage In Nanostructured Materials By Modifying sp² Covalent Bonds” by Young-Kyun Kwon, Seung-Hoon Jhi, Keith Bradley, Philip G. Collins, Jean-Christophe P. Gabriel, and George Grüner.

[0036] In these embodiments sp² bonds 24 are modified, for example, by forming non-planar structures, by introducing various defects, and by introducing different elements into nanostructured storage material 10. In these embodiments the so far passive additional p electron may hybridize to a small degree with the elections, which formed the sp² configuration. Therefore, in these embodiments sp² bond 24 may acquire a small sp³ character. Thus, in these embodiments the covalent bonds of nanostructured storage material 10 may be only substantially sp² bonds.

[0037] FIGS. 3A and 32B illustrate some embodiment for storing hydrogen in an sp² bonded nanostructured storage material 10 with a planar triangular lattice that has been deformed into a tubular structure, or nanotube.

[0038] FIG. 3A shows a perspective view of the nanotube without the hydrogen molecules, where the chemical composition of nanostructured storage material 10 is of the AB type, an example of which is boron nitride. Light elements 16 and light elements 20 of nanostructured storage material 10 are shown with bigger and smaller empty circles, respectively. Light elements 16 and 20 are coupled with sp² bonds 24.

[0039] FIG. 3B shows a perspective view of the nanotube, wherein hydrogen molecules 28 are shown in their bonding position. In some embodiments hydrogen molecules 28 are positioned over the center of the hexagons of the triangular lattice, in other embodiments hydrogen molecules 28 are positioned over a bond or over an atom of the triangular lattice. In some embodiments there is one energetically favored position for hydrogen molecules 28, in other embodiments there are several substantially equivalent positions. In some embodiments the orientation of hydrogen molecules 28 can be parallel to the local plane of the triangular lattice, in other embodiments the orientation can be perpendicular, or it can make some other angle with the local plane of the triangular lattice.

[0040] FIGS. 4A and 4B illustrate some embodiment for storing hydrogen in an sp² bonded nanostructured storage material 10 with a planar triangular lattice that has been deformed into a cage structure, or nanocage.

[0041] FIG. 4A shows a perspective view of the nanocage without the hydrogen, where the chemical composition of nanostructured storage material 10 is of the AB type, an example of which is boron nitride. Light elements 16 and light elements 20 of nanostructured storage material 10 are shown with bigger and smaller empty circles, respectively. Light elements 16 and 20 are coupled with sp² bonds 24.

[0042] FIG. 4B shows a perspective view of the nanocage, wherein hydrogen molecules 28 are shown in their bonding position. In some embodiments hydrogen molecules 28 are positioned over the center of the hexagons of the triangular lattice, in other embodiments hydrogen molecules 28 are positioned over a bond or over an atom of the triangular lattice. In some embodiments there is one energetically favored position for hydrogen molecules 28, in other embodiments there are several substantially equivalent positions. In some embodiments the orientation of hydrogen molecules 28 can be parallel to the local plane of the triangular lattice, in other embodiments the orientation can be perpendicular, or it can make some other angle with the local plane of the triangular lattice.

[0043] In further embodiments hydrogen can be stored in other forms of nanostructured storage material 10. A non-exhaustive list of possible forms of nanostructured storage material 10 includes:

[0044] nanofibers of the following kinds: turbostatic, highly oriented, twisted, straight, curled and rigid;

[0045] nanotubes of the following kinds: single walled, double walled, multi walled, with zig-zag chirality, or a mixture of chiralities, twisted, straight, bent, kinked, curled, flattened, and round;

[0046] ropes of nanotubes, twisted nanotubes, braided nanotubes;

[0047] small bundles of nanotubes (with a number of tubes less than ten), medium bundles of nanotubes (with a number of tubes in the hundreds), large bundles of nanotubes (with a number of tubes in the thousands);

[0048] nanotorii, nanocoils, nanorods, nanowires, nanohorns;

[0049] empty nanocages, filled nanocages, multifaceted nanocages, empty nanococoons, filled nanococoons, multifaceted nanococoons;

[0050] thin nanoplatelets, thick nanoplatelets, intercalated nanoplatelets, with thickness of about 0.3 nm to about 100 nm, and lateral size of about 5 nm to about 500 nm.

[0051] All these structures can assume heterogeneous forms. Heterogeneous forms include structures, where one part of the structure has a certain chemical composition, within another part of the structure has a different chemical composition. An example is a multi walled nanotube, where the chemical composition of the different walls can be different from each other.

[0052] Heterogeneous forms also include different forms of nanostructured storage material 10, where more than one of the above listed forms are joined into a larger irregular structure. Finally, all above d materials can have cracks, dislocations, branches or other imperfections.

[0053] It is understood that the scope of the invention extends to all the above listed and described forms of nanostructured storage material 10.

[0054] Economic and practical considerations prefer hydrogen storage systems that are light. In particular, light storage systems have higher storage efficiency in the sense that the weight % of the stored hydrogen is higher in light storage systems. Therefore, embodiments of the present invention include nanostructured storage material 10 composed of light elements. Suitable light elements include elements of the second and third rows of the periodic table: Be, B, C, N, O, F, Mg, P, S, and Cl. The storage efficiency of nanostructured storage material 10 can be enhanced by combining the listed suitable light elements. While Al and Si containing nanostructures, such as zeolites, have been explored before for storage purposes, in some embodiments of the invention Al and Si can be combined with the above light elements.

[0055] Some embodiments can contain elements from other rows of the periodic table as well. Some of these elements can be introduced deliberately to enhance a desired property. Other elements may be a residue from the production process, for example, a catalyst. Therefore, it is understood that embodiments of the invention may contain heavier elements in some concentration.

[0056] Some embodiments of the invention include B_xC_yN_z, where x, y, and z are small integers. Making of this material is described, for example, in “Pyrolytically Grown Arrays of Highly Aligned B_xC_yN_z Nantubes”, by W. -Q. Han, J. Cumings, and A. Zettl in Applied Physics Letters, vol. 78, p. 2769 (2001), and in U.S. Pat. No. 6,231,980 “B_xC_yN_z nanotubes and nanoparticles,” by M. Cohen and A. Zettl, which publication and patent are hereby incorporated in their entirety by this reference.

[0057] Some embodiments of the invention include BN. Making of this material is described, for example, in “Mass Production of Boron Nitride Double-wall Nanotubes and Nanococoons,” by J. Cumings and A. Zettl in Chemical Physics Letters, vol. 316, p. 211 (2000), hereby incorporated in its entirety by this reference.

[0058] Some embodiments of the invention include BC₂N. Making of this material is described, for example, in “Synthesis of B_xC_yN_z Nanotubules,” by Z. Weng-Sieh, K. Cherrey, N. G. Chopra et al., in Physical Review B, vol. 51, p. 11229 (1995), hereby incorporated in its entirety by this reference.

[0059] Some embodiments of the invention include MgB₂. Making of this material is described, for example, in “Superconducing MgB₂ Nanowires,” by Y. Wu, B. Messer, and P. Yang in Advanced Materials, vol. 13, p. 1487 (2001), hereby incorporated in its entirety by this reference.

[0060] Some embodiments of the invention include Be₃N₂. Making of this material is described, for example, in “Die Struktur Einer Neuen Modifikation von Be₃N₂, ” by P. Eckerlin and A. Rabenau in Angewandte Chemie, vol. 304, p. 218 (1960), hereby incorporated in its entirety by this reference.

[0061] Some embodiments of the invention include BeB₂. Making of this material is described, for example, in “Absence of Superconductivity in BeB₂, ” by I. Felner in Physica C, vol. 353, p. 11 (2001), hereby incorporated in its entirety by this reference.

[0062] Some embodiments of the invention include B₂O. Making of this material is described, for example, by H. Hall and L. Compton in Inorganic Chemistry, vol. 4, p. 1213 (1965), hereby incorporated in its entirety by this reference.

[0063] Some embodiments of the invention include elemental boron. Making of this material is described, for example, by S. La Placa, P. Roland, and J. Wynne in Chemical Physics Letter vol. 190, p. 163 (1992), hereby incorporated in its entirety by this reference.

[0064] Some embodiments of the invention include standard materials that are listed in the Chemical Abstract Service (CAS) at the web site: www.cas.org: 1

[0065] In the above embodiments of the invention the hydrogen atoms bonds to nanostructured storage material 10 by physisorption. Therefore, the present invention differs from U.S. Pat. No. 5,653,951, which describes an invention, where “ . . . nanostructures of the present invention store hydrogen by chemisorbing molecular hydrogen in the interstices of the nanostructure” (col. 3, 11.40-42).

[0066] There are several crucial differences between physisorption and chemisorption. The following comparative table is assembled according to the Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, (2001): 2

[0067] As Table 2 states, chemisorption is a type of adsorption, where an adsorbate is bound to a surface by the transfer of electrons, forming a chemical bond between the adsorbate and atoms of the surface. In contrast, physisorption is a type of adsorption, where an adsorbate is bound to a surface by physical interactions without the transfer of electrons. Physical interactions, giving rise to physisorption, include, but are not restricted to, van der Waals interactions. Van der Waals interactions are operational when the neutral atoms or molecules of the adsorbate and the surface polarize each other, and the polarized atoms or molecules attract each other at some distance.

[0068] Accordingly, chemisorptive bonds are strong and physisorptive bonds are weak. This difference manifests itself in the kinetics of the dissolution of the bonds, or desorption. Chemisorptive bonds are dissolved by an activated process, i.e., by thermal activation over an activation energy barrier, which is considerable. For this reason dissolution of chemisorbed bonds proceeds slowly and is not adiabatically reversible.

[0069] In contrast, physisorptive bonds are dissolved by a non-activated process. The activation energy is small, in many cases immeasurably small. Hence physisorptive bonds dissolve rapidly and reversibly.

[0070] U.S. Pat. No. 5,653,951 discusses in great detail the irreversible aspects of desorption of that invention. The irreversibility of desorption of that invention is illustrated in FIGS. 1A, 1B, 2B, 3A, and 3B by the fact that the sorption and desorption curves do not coincide. Such irreversibility is strong evidence for the chemisorptive nature of hydrogen bonding in U.S. Pat. No. 5,653,951.

[0071] FIG. 5 illustrates the coverage fraction, i.e., the fraction of the adsorbent surface, which is covered by hydrogen, as a function of pressure at a fixed temperature. The solid sorption curve indicates the results for starting at a low pressure value, for example, 10⁻³ atm, and measuring the coverage fraction while increasing the pressure. The dashed desorption curve indicates the results for starting at a high pressure value, for example, 1 atm, and measuring the coverage fraction while decreasing the pressure.

[0072] In contrast to U.S. Pat. No. 5,653,951, as shown in FIG. 5, the sorption and desorption curves of the present invention substantially coincide, indicating that the adsorption occurs by physisorption.

[0073] The above embodiments can be manufactured by different techniques. The two main acts of manufacturing are the making of nanostructured storage material 10 and the subsequent purifying of nanostructured storage material 10.

[0074] According to some methods of the invention, the making of nanostructured storage material 10 starts by providing the bulk material with the desired chemical composition in polycrystalline or micrograined form. The bulk material is then loaded into a ball mill. The balls of the ball mill can be stainless steel balls or hard ceramic balls, with a diameter of, for example, about 3 mm. The material can be milled for an extended period, for example, 24 hours. The milling can take place in air, in vacuum, or in reactive gases. The resulting material will contain some nanostructured storage material 10, which can be separated by subsequent purifying.

[0075] According to some other methods of the invention, the making of nanostructured storage material 10 starts with providing a bulk material in powder form. In some embodiments the bulk material has the desired chemical composition, in others the bulk material contains some of the desired ingredients. The powdered bulk material is pressed into a shape, suitable for functioning as an electrode, an example of such a shape being a rod. The rod is then used as an electrode to construct an arc chamber. The construction of such an arc chamber is described, for example, in “A Mass Production of Boron Nitride Double Wall Nanotubes and Nanocacoons,” by J. Cumings and A. Zettl in Chemical Physics Letters, vol. 316, p. 211 (2000). The arc chamber can be filled up with inert gases, for example, helium. In some embodiments the arc chamber can be filled up by one or more reactive gas. For example, beryllium nitride, Be₃N₂, could be made in an arc chamber whose electrode contains beryllium and where the chamber contains nitrogen gas.

[0076] Subsequent purifying of nanostructured storage material 10 can be performed by many methods. Some purifying methods are described by A. Dillon et al. in Advanced Materials, vol. 11, p. 1354 (1999), G. Duesberg et al. in Applied Physics A, vol. 67, p. 117 (1998), K. Shelimov et al. in Chemical Physics Letters, vol. 282, p. 429 (1998), J. Tak et al. in Chemical Physics Letters, vol. 344, p. 18 (2001), P. Young et al. in Carbon, vol. 39, p. 655 (2001), which publications are hereby incorporated in their entirety by this reference. Purifying methods are capable of extracting nanostructured storage material 10 of a particular class and form, for example, corresponding to the above list of nanostructured storage materials 10.

[0077] FIG. 6 illustrates an embodiment of a hydrogen storage system 100 according to the invention. An outer container 104 contains vacuum in outer region 108 to insulate the internal parts of hydrogen storage system 100 from the heat of the environment. The vacuum in outer region 108 can be controlled through vacuum valve 110. A pump can be coupled to vacuum valve 110 to reduce the pressure in outer region 108. In some embodiments the pressure in outer region 108 is between about 10⁹ atm and 10⁻¹ atm, for example, 10⁻⁶ atm.

[0078] Middle container 112 contains a cooling substance 116 to provide cooling of hydrogen storage system 100. Cooling substance 116 can be, for example, liquid nitrogen.

[0079] Cooling systems utilizing liquid nitrogen have multiple advantages over systems utilizing liquid helium. Liquid nitrogen is much cheaper per liter than liquid helium. Nitrogen becomes a liquid at 77 K, whereas helium becomes a liquid at 4.2 K. It requires much less energy to cool a system to a temperature of 77 K, than to a temperature of 4.2 K. It also requires a much simpler, and therefore lighter cooling apparatus to maintain a temperature of 77 K, than to maintain a temperature of 4.2 K.

[0080] Cooling valve 114 is used to control the cooling substance. For example, cooling substance 116 can be supplied through cooling valve 114, and the evaporated excess cooling substance 116 can be released through cooling valve 114. In embodiments that use different hydrogen storage materials other cooling substances 116 may be utilized.

[0081] Inner container 120 contains nanostructured storage material 10. Nanostructured storage material 10 can be any embodiment described above or a combination thereof. In some embodiments nanostructured storage material 10 substantially fills up inner container 120. In other embodiments nanostructured storage material 10 is combined with a hydrogen distribution system, which provides paths for the hydrogen to flow efficiently across the volume of nanostructured storage material 10. The hydrogen distribution system can be, for example, a hierarchical network of tubes of varying diameters to facilitate the efficient flow of hydrogen.

[0082] A hydrogen valve 126 is used to control the hydrogen gas. Hydrogen storage system 100 can be filled by coupling hydrogen valve 126 to a hydrogen container through a pump.

[0083] FIG. 7 illustrates the hydrogen storage capacity in percents in nanostructured storage material 10 as a function of the temperature. The amount of stored hydrogen is normalized with the amount stored at zero temperature. The curves refer to pressures of 1 atm, 10 atm, and 100 atm. FIG. 7 illustrates that as the temperature is increased from a low value, hydrogen starts the desorption at a relatively well defined desorption temperature T_D. The value of T_D depends on the pressure, as shown. When the temperature is raised to about 120% of T_D at 1 atm, and to about 150% of T_D at 100 atm, substantially all hydrogen is released. The value of T_D is about 60 K for carbon nanostructures and higher for boron nitride nanostructures. In FIG. 7 the temperature is given relative to the desorption temperature T_D at 1 atm, T_D(1 atm).

[0084] The amount of hydrogen adsorbed in a storage material can be characterized by the percentage wise weight increase of the storage system caused by the adsorption of hydrogen, in units of weight %.

[0085] Several papers addressed the storage of hydrogen in nanostructures at ambient temperatures. For most nanostructured materials the desorption temperature is well below ambient temperature, in accordance with the fact that physisorptive bonds are weak. Therefore, the amount of hydrogen stored in these nanostructures at ambient temperature is rather small. For example, M. Ashraf Imam and R. Loufty report in “Hydrogen Adsorption of Different Types of Nanotubes,” on p. 40 of the Procedings of NT'01, the International Workshop of on the Science and Applications of Nanotubes, hereby incorporated in its entirety by this reference, that single walled nanotubes adsorb hydrogen in an amount between about 0.30 weight % and about 0.50 weight %.

[0086] In contrast, embodiments of the invention store hydrogen in cooled storage systems at temperatures below the desorption temperature, enabling the storage of much larger quantities. In the above units hydrogen adsorption below the desorption temperature in some embodiments is between about 3 weight % and about 27 weight %, for example, 7.5 weight %. Operating hydrogen storage system 100 below the desorption temperature enhances the storage capacity by a factor of 30 or more in comparison to operating the hydrogen storage system 100 at ambient temperatures.

[0087] At the same time embodiments of the invention are operated at or above liquid nitrogen temperatures, and thus do not require the use of liquid helium for cooling purposes. As discussed above, cooling systems using liquid nitrogen have many advantages over cooling systems using liquid helium.

[0088] Storage systems that advantageously use liquid nitrogen as cooling substance 116 require nanostructured storage materials 124 that have a desorption temperature above 77 K. The desorption temperature of nanostructured storage material 10 depends on its chemical composition. In particular, while pure carbon nanostructures typically have desorption temperatures below 77 K, embodiments that use nanostructured storage materials 124 formed from a combination of light elements have desorption temperatures well above 77 K. For example, boron nitride has a desorption temperature which is about 30% higher than that of carbon.

[0089] Storage systems with higher desorption temperatures require less energy for their operation. In particular, storage systems with desorption temperatures at or above the ambient temperature do not require a cooling system, making the storage system much lighter. Suitable selection of the chemical composition of nanostructured storage material 10 may increase the desorption temperature to ambient temperature.

[0090] The operation of hydrogen storage system 100 includes filling up the system with hydrogen and recovering stored hydrogen from the system.

[0091] Hydrogen storage system 100 can be filled up by coupling hydrogen valve 126 to a hydrogen container through a pump. Nanostructured storage material 10 is cooled by cooling substance 116 to temperatures below the desorption temperature. When the pressure of the pump is raised to a suitable value, hydrogen from the hydrogen container will be pumped into inner container 120, where it will adsorb to nanostructured storage material 10. Suitable pressure values can lie in the range of, for example, about 1 atm to about 20 atm.

[0092] Hydrogen can be recovered from hydrogen storage system 100 by various methods. Some embodiments recover the hydrogen by heating nanostructured storage material 10 with heater 130. Heater 130 can be, for example, a resistor, driven by a current. The longer the current is applied to heater 130, or the more current is applied to heater 130, the higher the temperature of nanostructured storage material 10 will rise. As the temperature rises above the desorption temperature, hydrogen will desorb from nanostructured storage material 10, and can be recovered through hydrogen valve 126. In some other embodiments simply the leakage heat, leaking into inner container 120 from the outside, can be used to drive the desorption of hydrogen.

[0093] Although the various aspects of the present invention have been described with respect to certain embodiments, it is understood that the invention is entitled to protection within the full scope of the appended claims.