DE102008034666B4 - Process for the activation of metal hydride particles or of particles of metals which are metal hydride precursors - Google Patents

Abstract

A method for activating metal hydride particles or particles of metals that are metal hydride precursors, the particles having surface films which inhibit a release of hydrogen from the particles or an absorption of hydrogen into the particles, the method comprising: a) suspending the particles in supercritical carbon dioxide or liquid nitrogen and b) subjecting the liquid-particle mixture to cavitation.

Description

This filing claims priority based on the provisional filing 60 / 952.293 entitled "Activation of Metal Hydrides," which was filed July 27, 2007 and is incorporated herein by reference.

TECHNICAL AREA

This disclosure relates to the treatment of surfaces of metal hydride particles to activate the particles for storage or release of hydrogen. More specifically, this disclosure relates to such activation of metal hydride particles in which they are suspended in a suitably inert liquid, namely supercritical carbon dioxide or liquid nitrogen, and subjected to cavitation to produce hydrogen-permeable surfaces.

BACKGROUND OF THE INVENTION

In the context of this disclosure, metal hydrides are elements and alloys or intermetallic compounds of elements that are capable of reversibly absorbing hydrogen. As used herein, the terms metal hydride and metal hydride alloy refer to both metal hydrides in a non-hydrogenated state, sometimes referred to as hydrogenating metal alloys, and metal alloys in a hydrogenated state which include the metal alloys with hydrogen. These metal hydride alloys may take the form of solid solution type alloys or intermetallic compounds. The hydrogen is built into a matrix of metal atoms for storage in solid states. The matrix may have a lattice of a metal crystal structure and the hydrogen atoms are inserted between the metal atoms.

Metal hydride alloys are useful, for example, for the storage of hydrogen for fuel cells and other hydrogen-consuming drive systems for motor vehicles. Hydrogen can be absorbed in a non-hydrogenated metal composition (metal hydride precursor) by cooling the precursor material to a suitable, relatively cold storage temperature and contacting it with hydrogen gas under a suitable, usually relatively high pressure. The hydrogenated metal hydride material is stored (often in a vehicle) until hydrogen is needed. The metal hydride is then heated and hydrogen is released into a delivery system to provide hydrogen to a device (often an on-board device) that will utilize it.

Examples of combinations of metal (s) and corresponding hydrides include Pd and PdH ₀ , ₆ , LaNi ₅ and LaNi ₅ H ₇ , ZrV ₂ and ZrV ₂ H _5.5 , FeTi and FeTiH2, and Mg ₂ Ni and Mg ₂ NiH ₄ .

In practice, many metal hydride precursors cannot easily absorb and store hydrogen. The particles need pre-treatment before they can absorb hydrogen. Pretreatment (sometimes called "activation") involves removing oxide films (or other hydrogen impermeable films) on the metal particles or breaking some of the particles to expose unoxidized surfaces for hydrogen absorption. Such methods have included cooling the particles, pressurizing with hydrogen, heating, and external pressurizing the particles to chemically remove oxide barriers to hydrogen absorption. It was sometimes necessary to go through it again.

There is a need for less costly and time consuming methods of activating hydrogen storage materials.

From the U.S. 5,882,623 A a method for inducing the desorption of hydrogen from a metal hydride is known, in which the metal hydride, for example by treatment with ultrasound, is supplied with sufficient energy to induce the hydrogen desorption by an endothermic reaction.

SUMMARY OF THE INVENTION

1. a) Suspendieren der Partikel in überkritischem Kohlendioxid oder Flüssigstickstoff und
2. b) Unterziehen des Flüssigkeits-Partikel-Gemisches einer Kavitation.

The present invention relates to a method for activating metal hydride particles or particles of metals which are metal hydride precursors, the particles having surface films which inhibit a release of hydrogen from the particles or an absorption of hydrogen into the particles, the method comprising:

1. a) suspending the particles in supercritical carbon dioxide or liquid nitrogen and
2. b) subjecting the liquid-particle mixture to cavitation.

Particles of a metal hydride material or metal hydride precursor can be evaluated to determine whether they are susceptible to absorption or desorption of hydrogen. If it is determined that surfaces of the particles are occluded or otherwise prevent hydrogen exchange, the particles can be treated as follows.

Metal particles (or metal hydride particles) are dispersed in an inert liquid (namely supercritical carbon dioxide or liquid nitrogen) and the particle-liquid mixture is subjected to cavitation by a suitable cavitation actuator. An ultrasonic generator is often suitable for generating cavitation. Cavitation is the sudden formation and breakdown of low pressure bubbles in the liquid, typically by mechanical forces. The liquid composition must tolerate the application of ultrasonic mechanical energy and should not cause interference with subsequent hydrogen absorption by treated particles. The mixture of liquid and hydride particles is contained in a vessel for cavitation of the mixture. The cavitation of the liquid is carried out to cause the oxide (or other restraining coating) on the surfaces of the particles to wear away. Cavitation can also cause the metal or metal hydride particles to break to expose fresh, unsealed surfaces for absorption or release of hydrogen.

The vessel can be designed in such a way that a protective atmosphere is provided when the cavitation mixture requires it. Furthermore, the vessel can be designed for a release of hydrogen from the treated particles or for an injection of hydrogen to be absorbed by the particles. A slight increase in temperature can be caused by the cavitation treatment. The vessel can be equipped with a system for a desired temperature control of the liquid / particle mixture during the cavitation process.

The suitable liquid treatment media are supercritical carbon dioxide and liquid nitrogen. These fluids will require containment vessels capable of maintaining temperatures and pressures in order to maintain these fluids in a cavitation mode. One advantage of these cavitation liquids is that they can easily be vaporized from the activated metal or hydride particles.

Upon completion of the cavitation activation of the particles, the liquid can be separated from the activated particles and the particles can be used in their hydrogen storage function. Hydrogen can be added to or removed from the activated particles. In other embodiments of the invention, hydrogen can be added to or removed from the activated particles while the particles are still suspended in the liquid.

Some metal-metal hydride combinations can store or release copious amounts of hydrogen by varying the hydrogen pressure to which they are exposed in a closed volume. Some metal-hydride combinations release hydrogen under sufficiently high hydrogen pressure to facilitate transport of the released hydrogen from a hydrogen storage material to a nearby hydrogen consuming device. The practice of this invention is applicable to many metal hydrides, including such high pressure metal hydrides.

Other objects and advantages of the invention will become apparent following a further description of preferred embodiments of the invention.

Figure list

• 1 is a hydrogen pressure-hydrogen content isotherm for a typical metal hydride of the type AB ↓ 5 ↓ In 1 the samples were held at constant temperatures of 0 ° C and 40 ° C respectively while the hydrogen gas pressure was increased from very low pressure to about 4 megapascals and then slowly dissolved.
• 2 Figure 13 is a schematic representation of a cavitation device for activation of metal hydride particles or metal particles that are precursors for metal hydrides.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The most promising chemical reactions for on-board or built-in H ₂ storage are equilibrium _{reactions that store and release H 2} alternately according to the phase, temperature and pressure of the system. One class of such chemical reactions are those that involve metal hydrides. Metal hydrides undergo reversible chemical reactions that either absorb or release hydrogen under temperature and pressure conditions sometimes appropriate for on-board vehicles. The starting metals of metal hydrides typically take any of the following forms: A, AB ₅ , AB ₂ , AB, A ₂ B, where A and B are typically a mixture of transition metals or a mixture of transition metals with either rare earth metals or alkaline earth metals. Table 1 shows examples of metal hydrides and their hydrogen storage properties.
Table I: Examples of metal hydrides with known hydrogen storage properties. Type metal Hydride structure Mass% H ₂ P _eq , T A. Pd PdH _0.6 Fm3m 0.56 0.02 bar, 298K FROM ₅ LaNi ₅ LaNi ₅ H ₇ P6 / mmm 1.37 2 bar, 298 K FROM ₂ ZrV ₂ ZrV ₂ H _5.5 Fd3m 3.01 10 ^-8 bar, 303 K FROM FeTi FeTiH ₂ Pm3m 1.89 5 bar, 303 K A ₂ B Mg ₂ Ni Mg ₂ NiH ₄ P6222 3.59 1 bar, 555 K

The formation of a metal hydride from the reaction of hydrogen with a parent metal (s) proceeds via a set of well-described processes. Hydrogen molecules first dissociate on the surface before the dissociated hydrogen dissolves in the host metal, being in the interstitial spaces of the host metal's crystal structure. But molecular hydrogen must be able to contact the surface of pure metal hydride for dissociation and dissolution to take place. In other words, the surface of a metal hydride precursor material must be free of films that inhibit hydrogen absorption.

With a higher hydrogen pressure, the concentration of dissolved hydrogen increases and the distance between the hydrogen atoms decreases with an increasing number of filled interstitial spaces. At any given temperature and pressure, hydride formation will continue until the material is fully hydrogenated, at which point an increasing concentration of hydrogen gas cannot generate additional hydride and the pressure increases. These steps can be represented with pressure-composition isotherms, an example of which is shown in 1 is shown. In these examples at 0 ° C and 40 ° C, it can be seen that higher hydrogen pressures are required to achieve equivalent hydrogen absorption in the hydrogen storage composition. At even higher pressures, however, different interstitial positions can be occupied, with consequent formation of different phases of the metal hydride. For many of the known metal hydrides, the thermodynamic properties of the interstitial spaces that are accessible for hydrogen storage at higher hydrogen pressures are either not known or remain to be explored.

The examples of metal hydrides shown in Table I are of the binary and ternary types, typically expressed in the forms AH _x , A ₂ BH _x, and AB _n H _x . Quaternary metal hydrides also exist, exemplified by the general form AB _n C _m H _x . Consequently, there is an almost limitless number of potential metal hydrides that can be explored.

An example of a classic metal hydride shown in Table I is LaNi ₅ H ₇ . The formation of this hydride occurs at moderate pressures and temperatures, as shown in Table I. This hydride has a relatively low heat of formation; therefore, the energy required for hydrogen release is low. It also releases hydrogen at relatively low temperatures, allowing waste heat from a fuel cell to be used to release the hydrogen. As a result, the uptake and release of hydrogen occurs under conditions favorable to on-board hydrogen storage in vehicles, and the harmful loss associated with the release of hydrogen from this hydride is small.

Thus, metal hydrides, which are composed of many different combinations of different metals, are available and are, in view of their potential to store hydrogen, developed. But sometimes these materials need to be “activated” before their potential to store and release hydrogen is fully realized. The activation process is often associated with removing oxide layers from the hydride or hydride precursor material and / or exposing freshly unoxidized parts of the material. The activation processes that have previously been used have been complex, time consuming and expensive. For example, one process uses temperature and pressure cycling to activate a high pressure metal hydride. In the process, each cycle consists of cooling the metal hydride to -190 ° C and pressurizing it with hydrogen to 175 bar, then heating the metal hydride to 350 ° C while evacuating the chamber to vacuum. Several such heating / pressurizing and cooling / evacuating cycles are required for activation. Another activation process consisted of sustained heating of the metal hydride to 1200 ° C.

Irradiating a liquid with, for example, ultrasound induces cavitation. Ultrasonically induced cavitation creates bubbles in the liquid that can reach 5000 K and pressures of up to 1000 atmospheres. These bubbles will form on surfaces of solids that are suspended or within the cavitation medium. It has been shown that bubbles generated by cavitation undergo growth during cyclic expansion and compression due to the dilution and compression of the ultrasonic waves in liquid media. Bubbles eventually become unstable in size and subject to violent collapse on the solid surface. During the collapse of the bladder, the liquid medium forms a jet of liquid that is directed at the solid surface and reaches speeds of hundreds of meters per second. It has been described that such liquid jets have "shape-charged" effects, which remove the surface of the solid. The forces of this collapse are sufficient to remove oxide layers from some metals and to break brittle particles.

Consequently, this effect is precisely the result that is necessary for the activation of metal hydrides and their precursors. This invention uses supercritical carbon dioxide or liquid nitrogen as the medium in which cavitation is induced by an ultrasonic generator. Metal or metal hydride particles are suspended in the cavitation medium. Bubbles generated by cavitation in the medium form on the particle surfaces, and the implosive collapse of these bubbles on the surfaces of the particles causes the oxide layer to be removed, thereby exposing fresh, non-oxidized surfaces of the hydrogen storage materials. The surfaces of the particles are also broken by the force of the liquid jet against the surfaces, thereby increasing their surface area. The removal of oxide layers and the increased surface area serve to activate the materials for hydrogen uptake and release.

2 Fig. 10 illustrates a laboratory cavitation device in which an ultrasonic generator is used to induce cavitation in a mixture of metal hydride precursor particles (and / or metal hydride particles) and liquid. With reference to 2 an ultrasonic generator contains an ultrasonic vibrator container 10 with a liquid bath 14th for transferring ultrasonic vibration energy to a double-walled cavitation vessel 12 . A metal hydride particle-liquid mixture 16 is in the cavitation vessel 12 contain. The top of the cavitation vessel 12 is with an airtight lid 18th sealed, which is carrying a thermometer 20th , a gas injection pipe 22nd for a gas inlet 24 (if desired) and a gas outlet 26th enables. The optional gas inlet 24 and gas outlet 26th can be used to provide a protective gas atmosphere, such as argon, over the contents of the cavitation vessel. Or the inlet and outlet can be used for admitting or removing hydrogen to and from the vessel 12 be used. The cavitation vessel 12 is double-walled and the temperature control of the vessel 12 is made using thermostat-controlled (thermostat 28 ) Liquid circulation lines 30th and 32 reached.

A suitable cavitation liquid, namely supercritical carbon dioxide or liquid nitrogen, is selected for the cavitation treatment of the hydrogen storage particles. The liquid does not adversely affect the particles during cavitation and is suitable for cavitation treatment at the desired treatment temperature. And the liquid is easily removed from the particles when the treatment is completed.

Cavitation vessels are designed or designed for a cavitation treatment of the metal hydride precursor particles or metal hydride particles. The cavitation treatment will often be performed on a batch of particles and the vessel will be shaped accordingly. But a cavitation reaction could also be carried out in a flow-through vessel on a semi-continuous or continuous basis. The size of the vessel is set so that it offers space for the cavitation liquid and the particle mass. As explained above, the cavitation vessel may have to be heated or cooled depending on the selection of the cavitation liquid and the temperature at which the cavitation process is carried out. Precautions are taken to admit the untreated liquid-particle mixture into the vessel and to remove the activated mixture from the vessel under a suitable, non-oxidizing atmosphere. Provision can also be made for admitting a protective atmosphere into the vessel or for admitting or removing hydrogen if such a treatment is carried out while the cavitation mixture is in the cavitation vessel.

An ultrasonic generator (or other cavitation device) is used in combination with the cavitation vessel. The ultrasonic generator can act on a wall or surface of the vessel, as in FIG 2 is shown to induce ultrasonic frequency vibrations in the cavitation mixture in the vessel. In another embodiment, the ultrasound generator can have a horn or some other transmitter, which extends into the cavitation vessel, for direct contact with the liquid-particle mixture.

While an ultrasonic generator is a suitable tool for inducing cavitation in an activation process for metal hydrides, other cavitation-generating agents can be used for such an activation treatment. Cavitation can also be induced by pumps, propellers or other techniques that cause local reductions in the pressures of the cavitation medium to values below the vapor pressure of the medium.

Embodiments of the invention are described by some illustrations, which are not intended to restrict the scope of the invention.

Claims (4)

A method for activating metal hydride particles or particles of metals which are metal hydride precursors, the particles having surface films which inhibit a release of hydrogen from the particles or an absorption of hydrogen into the particles, the method comprising: a) suspending the particles in supercritical carbon dioxide or liquid nitrogen and b) subjecting the liquid-particle mixture to cavitation. Procedure according to Claim 1 , in which the liquid-particle mixture is subjected to ultrasonic frequency vibrations to subject the mixture to cavitation. Procedure according to Claim 1 , in which a cavitation is started in a liquid-particle mixture at a temperature above room or ambient temperature, the liquid being supercritical carbon dioxide. Procedure according to Claim 1 , in which cavitation is started in a liquid-particle mixture at a temperature below room or ambient temperature, the liquid being liquid nitrogen.

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