FIELD OF THE INVENTION

The invention relates to process for loading solid substances with hydrogen, and to substances loaded with hydrogen, obtainable by the process.

BACKGROUND OF THE INVENTION

In recent years there is a developing interest in the possibility to manufacture vehicles, such as cars, that will be fueled with hydrogen instead of gasoline. Since hydrogen has high tendency to leak and, when leaked, to explode, and since hydrogen pressurized in an ordinary tank or container is unsafe under automobile crash events, the problem of hydrogen storage is one of the most serious problems to be solved in order to provide such a vehicle.

The main storage solutions suggested until now include physical storage, which involves development of tanks for either compressed or liquefied hydrogen; irreversible chemical storage, which involves storage materials that produce hydrogen in situ and should be physically replaced once they are exhausted; and reversible chemical storage, which involves storing hydrogen inside solid materials that should be replaced or recharged once the hydrogen stored in them is fully consumed.

State of the art reversible hydrogen storage materials hold up to 6%w/w hydrogen.

The present invention provides reversible hydrogen storage means capable of holding at least 10% w/w hydrogen.

SUMMARY OF THE INVENTION

According to one of its aspects, the present invention provides a process for producing a hydrogen loaded substance containing at least 10% w/w hydrogen, the process including exposing a heat steady polycrystalline solid substance to hydrogen pressure of at least 500 Atm at a temperature of at least 1000 ⁰ C.

Preferably, the pressure does not exceed 1500 Atm, and the temperature does not exceed 2500°C. The preferred temperature range is

2000 - 2500 ⁰ C and the preferable pressure range is 1000 -1500 Atm. Gas mixtures may also be used for applying the pressure, and in such a case, the above pressure values are of the partial pressure of hydrogen.

In the present description and claims, a solid is said to be heat steady if it remains solid under the pressure and temperature used in the process.

Solids with large surface area are preferable, accordingly, solids may be monolithic, but powders, granulated, porous, or spongy solids are preferable. Preferable solids are those having large interstructural area.

Interstructural area is said to be large if it is at least about 10 ⁴ to 10 ⁵ m ² /g.

Polycrystalline materials are generally made of crystals arranged in grains. Without being bound to theory, it was found by the inventors that under the specified temperature and hydrogen pressure, heat steady polycrystalline solid substances are expected to adsorb hydrogen at least in the following three kinds of sites: (i) borders between grains, (ii) borders separating the crystals from each other inside a grain and (iii) inter-atomic spaces inside crystals. This unique combination of three adsorption sites is believed to be responsible for the outstandingly high hydrogen content that may be achieved in the process of the invention. Of the three adsorption sites, it is believed that inter-atomic spaces inside crystals is of particular importance.

Preferably the polycrystalline substances used according to the invention have crystals of a sub-micron size. Substances with average crystal size of less than 1 micron may be suitable, but preferred are substances where even the largest crystal they contain is sub-micronic. Average crystal size of 500nm is preferred, more preferred is average crystal size of lOOnm, and smaller average crystal size is still preferable.

Another feature of preferred polycrystalline substances is that it includes crystals with relatively high inter-atomic space, occupying at least

50% and up to 70% of the crystal volume. Non-limiting examples of such crystals are those having a lattice with simple cubic or tetrahedral structure. Still another feature of preferred polycrystalline substance is that it has a capability to absorb at least 2% hydrogen (w/w) to form compounds of metal hydrides under pressure of up to 20 atmospheres and at a temperature of 300-500 ⁰ C. Non-limiting examples of polycrystalline substances that fulfill this requirement are Mg ₅ Mg ₂ Ni, V, Ni ₅ La and LaC ₂ .

Non-limiting examples of compound families that at the polycrystalline state may serve as polycrystalline substance in accordance with the invention are the family of compounds made of carbon and transition metals, carbon and rare earth elements, or intermetalloids. Non- limiting examples of suitable members of these families are FeTi, FeNiTi, CaNiTi, Mg ₂ Cu, LaNi _4-7 , LaNiAl ₀₃ . _, LaNi ₅ , LaC ₂ , and NiC ₂ .

Optimizing the polycrystalline substance in accordance with the features mentioned above together with optimizing the temperature and the hydrogen pressure may result in hydrogen loaded substances having hydrogen content of 15% w/w or more, preferably 20% or more, more preferably 30% or more, and even up to 40%.

According to another aspect of the invention there is provided a polycrystalline heat steady solid substance loaded with hydrogen, wherein said hydrogen constitutes at least 10% w/w of the hydrogen loaded substance.

BRIED DESCRIPTION OF THE DRAWING Fig. 1 is a schematic illustration of a system for carrying out a method in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on insight gained by the inventors from theoretical calculations and simulations they preformed, from which the following picture immerges:

The major potential of increasing the contents of hydrogen inside an accumulating substance is in the possibility to implant hydrogen atoms inside inter-atomic space available in some crystalline materials, preferably, materials of simple lattice structure. It was found theoretically possible to implant hydrogen atoms into the lattice and simultaneously form a molecular layer of hydrogen, adsorbed on the borders between grains and the borders separating the crystals from one another inside grain. Such molecular layer turns, under high pressure, into a solid solution of hydrogen. Theoretically in this case it would be possible to accumulate up to 30 - 40% wt. of hydrogen in the material.

The present invention is based on the idea to provide such conditions as to allow larger amount of hydrogen to be implanted inside a lattice than is provided by the state of the art. Under the special environmental conditions proposed, hydrogen atoms penetrate into the lattice, first along the boundaries of the grains, then across the boundaries of the crystals and only than inside the crystal lattice, where it fills its volume as to form solid solution of the accumulating material with hydrogen.

In this last stage of hydrogen diffusion into the crystal, hydrogen atoms gradually capture imperfections of the lattice (dislocations, vacancies, micropores, etc.). In the process of diffusion along the boundaries of the grains and nano-crystals of the material the molecules of hydrogen interact with the atoms of the accumulating material to produce hydride compounds with them. These compounds, on the one hand, prevent hydrogen from penetrating further and, on the other hand, weaken bonds between the grains of crystalline material and threaten to destroy it.

The high pressure of the hydrogen, under which the process is carried out, prevents these destructions and the high temperature accelerates a process of significant growing of grains as to decrease the total area of the separation boundaries between the grains. Thanks to this process the hydrides of admixtures appearing between the grains are expelled onto the surface of the material. The firmness of the material increases and it endures internal

pressure of hydrogen after cooling the sample of the material and removing the external pressure.

Without being bound to theory, the inventors believe the above picture may provide a skilled person with insight as to preferred conditions and materials suitable for carrying out the method of the invention.

In order to see how the invention may be carried out in practice, an experimental system and procedure will now be described, by means of non- limiting example only.

First, lanthanum disks were carbidized to produce LaC ₂ in a process known per se, that includes subjecting the lanthanum disks to low pressure of hydrocarbons at 2500 ⁰ C for 10-30 minutes. This step may be omitted by using commercially available LaC ₂ .

Six plates, 1.5 g each, Of LaC ₂ thus obtained were subjected to pressure of 1000 Atm hydrogen and/or helium (as a simulator of hydrogen) at 1200 ⁰ C for 5 minutes and than cooled at a rate of 10°C/sec. After cooling, pressure was gradually lowered to atmospheric pressure. The gas-saturated LaC ₂ samples thus obtained were of 1.7 — 1.8g in weight. This weight increase corresponds to 15 - 20% of hydrogen accumulated in the solid LaC ₂ .

Subjecting the LaC ₂ to hydrogen under high temperature was carried out in the system described below in reference to Fig. 1.

The system includes a hydrogen reservoir 1 for providing the system with hydrogen at low pressure, a low pressure compressor 2, for raising the hydrogen pressure to about 2Atm, a high pressure compressing system 3 for further compressing the hydrogen to about 1000-1500Atm, and a thermal charging system 4, including an electric heater 5 and a cooler 5'. The cooler 5' is capable of providing cooled inert gas, such as argon, in a temperature of liquid nitrogen.

It is in the charging system 4 that LaC ₂ plates 6 are subjected to pressure and temperature according to the invention. Applying high temperature and pressure in the range required by the present invention is by itself known in the art, for instance, in the artificial diamond industry and in the metallurgy

industry, where suitable gasostats are in use. Such gasostats are manufactured, for instance by the American coAtmnies Natural Forge and Autoclass Engineering. Similar equipment may be used to carry out the present invention, however, care should be taken to construct the instruments (or at least those portions thereof that are in contact with hydrogen) from substances that are not attacked by hydrogen, and are not transparent to hydrogen, even under the high pressure and temperature used in a method according to the invention. Non- limiting example to a suitable substance is molybdenum-vanadium alloy, especially the alloy known in the art as VMl (BMl in Russian references).