Refrigeration – Storage of solidified or liquified gas – With sorbing or mixing
Reexamination Certificate
2001-12-11
2004-01-06
Esquivel, Denise L. (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
With sorbing or mixing
Reexamination Certificate
active
06672077
ABSTRACT:
BACKGROUND
1. Field of Invention
The invention relates to hydrogen storage systems, more particularly to the storage of hydrogen in systems that include nanostructures of combinations of light elements.
2. Description of Related Art
Hydrogen storage is the key unsolved problem of producing fuel cells for hydrogen-powered automobiles or portable energy devices. In particular, storing hydrogen in large quantities safely and in a light container has proved prohibitively difficult so far.
Several different techniques have been developed to tackle this problem. In some approaches hydrogen is stored in tanks under high pressure, for example, 300 atm. In other techniques hydrogen is liquefied at temperatures below 20 K with a helium-based cooling system. Both of these techniques pose problems for practical use in automobiles. All of the hydrogen is available for catastrophic release in an accident, raising the risk of explosion or fire. Furthermore, in order to store enough hydrogen to match the range of present day automobiles, the container has to have a volume of at least 50 gallons. Also, both in the high pressure technique and in the helium cooled technique the required containers are heavy, and therefore inefficient for storage. Finally, both techniques also consume a lot of energy for generating the high pressure or for liquefying the hydrogen.
Some other techniques adsorb hydrogen into solid materials. Several types of materials have been studied in this respect, including metal hydrides and glass microspheres. However, the materials investigated so far all have low hydrogen storage capacity, making them non-competitive with gasoline.
Hydrogen can be stored in carbon nanostructures, such as graphite and carbon nanofibers, according to the papers of A. Dillon et al. in Nature, vol. 386, p. 377 (1997), A. Chambers et al. in J. Phys. Chem. B vol. 102, p. 3378 (1998), and E. Poirier et al. in Int. J. of Hydrogen Energy, vol. 26, p. 831 (2001), and according to U.S. Pat. No. 5,653,951: “Storage of hydrogen in layered nanostructures,” by N. Rodriguez and R. Baker; and U.S. Pat. No. 4,960,450: “Selection and preparation of activated carbon for fuel gas storage,” by J. Schwarz et al. Furthermore, hydrogen storage in Al and Si containing zeolites and microporous materials has been explored previously.
Nanostructures can be defined as atomic structures that have a spatial extent of less than a few hundred nanometers in one, two, or all three dimensions. A class of nanostructures is formed by planar networks, sometimes referred to as layered compounds. Layered compounds are often formed by elements coupled with sp
2
bonds. The origin of the sp
2
bonds will be presented on the example of elements of the second row of the periodic table, including boron, carbon and nitrogen.
FIG. 1
shows an example of a second row element
4
coupled with sp
2
bonds, or orbitals,
8
to three other elements
12
. The s orbital of the second row elements is filled with two electrons, and the p orbitals are partially filled. For example, boron has one electron, carbon has two, and nitrogen has three electrons in the p orbitals. When the second row elements form chemical bonds, one of the s electrons is promoted into an empty p orbital—for example into the p
z
orbital in carbon, leaving only one s electron. This one s electron and two of the p electrons first hybridize into three s hybrid orbitals. The remaining p electrons—none in boron, one in carbon, and two in nitrogen—occupy an orbit that does not participate in the bonding. The three hybridized electrons repel each other, and hence form three sp
2
orbitals
8
as far as possible away from each other. An optimal configuration is when the three sp
2
orbitals
8
make 120 degrees with each other, defining a plane. Connecting several second row elements with planar sp
2
orbitals 8 spans the defined plane, thus forming the aforementioned planar networks.
Possible planar networks of the sp
2
bonded materials include triangular lattices. Large sections of a planar network can be deformed to create various nanostructures. Nanostructures that are based on sp
2
bonded triangular lattices include different classes of nanotubes, nanococoons, nanoropes, nanofibers, nanowires, nanohorns, and nanocages.
Storing hydrogen in sp
2
bonded nanostructures has the following advantages. Hydrogen, adsorbed to the nanostructures, desorbs over a range of temperatures, and thus it is not available for catastrophic release, for example, in case of an automobile accident. Furthermore, because of their large surface area, nanostructures are capable of adsorbing very large quantities of hydrogen, giving rise to a much higher weight % storage efficiency than the aforementioned high pressure and cooling techniques.
However, the above works have the following disadvantages. Typically they considered hydrogen storage at ambient temperatures, where the storage capacity fell far short of the theoretical value, making those works economically non-viable. Also, the works that considered storage at other temperatures reported insufficient storage efficiencies.
In particular, U.S. Pat. No. 5,653,951 considered hydrogen storage in carbon nanostructures, utilizing chemisorption. As described below in detail, chemisorption binds hydrogen to the carbon nanostructure by forming a chemical bond that is typically quite strong. Therefore, chemisorptive bonds can change the chemical composition and structure of the storage material itself. This is a drawback for storage applications, as the storage system has to be operated cyclically without structural degradation in order to be useful.
Also, because of the formation of chemical bonds, the hydrogen might be recovered from the storage material in an altered chemical form, for example, methane. This again reduces the usefulness of storage materials, which form chemisorptive bonds.
Therefore, there is a need for hydrogen storage systems that contain sp
2
bonded nanostructures, wherein the chemical composition of the nanostructure is selected to ensure high storage efficiency, the storage system operates at technically advantageous temperatures, and in particular wherein the mechanism of hydrogen adsorption is not chemisorption.
SUMMARY
In accordance with the invention, a hydrogen containing nanostructured storage material is provided, where the hydrogen is adsorbed to the nanostructured storage material by physisorption. The nanostructured storage material includes light elements, belonging to the second and third rows of the periodic table. More specifically, the light elements are selected from Be, B, C, N, O, F, Mg, P, S, and Cl. The chemical composition of the nanostructured storage material is such that the desorption temperature, at which hydrogen desorbs from the nanostructured storage material, is greater than the liquefaction temperature of nitrogen, 77 K. Some chemical compositions that give rise to a desorption temperature in excess of 77 K are: B
x
C
y
N
z
, BN, BC
2
N, MgB
2
, Be
3
N
2
, BeB
2
, B
2
O, B, BeO, AlCl
3
, Al
4
C
3
, AlF
3
,Al
2
O
3
, Al
2
S
3
, Mg
2
Si, Mg
3
N
2
, Li
x
N
y
, Li
x
S
y
, and Na
x
S
y
, where x, y, and z are integers.
The nanostructured storage material is formed as a layered network of light elements, coupled with covalent sp
2
bonds. The layered network can be a triangular lattice, a nanofiber, a nanoplatelet, a single walled nanotube, a multi walled nanotube, a nanocage, a nanococoon, a nanorope, a nanotorus, a nanocoil, a nanorod, a nanowire, and a fullerene. The layered network can also have a heterogeneous form, including a combination of the above structures, as well as embodiments where various parts of the network can have different chemical composition.
According to another embodiment of the invention, a hydrogen storage system is provided. The hydrogen storage system includes a container and a nanostructured storage material within the container, wherein the nanostructured storage material includes light elements, and the nanostructured storage material is capable of adsorbing hydrogen by
Bradley Keith
Collins Philip G.
Gabriel Jean-Christophe P.
Grüner George
Jhi Seung-Hoon
Drake Malik N.
Esquivel Denise L.
Nanomix Inc.
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