# Patent

## Title
Hydrogen storage materials

## Abstract

Hydrogen storage materials which are liquid metal alloys in their discharged state, thereby facilitating their recharging by reaction with hydrogen gas.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to and the benefit of U.S. provisional application Ser. No. 60/636,549 filed on Dec. 17, 2004. This application also claims priority to and the benefit of Canadian application Serial No. 2529433 filed on Dec. 7, 2005.


FIELD OF THE INVENTION
This invention relates generally to hydrogen storage materials and more specifically relates to hydrogen storage materials which are liquid metal alloys in their discharged state.
BACKGROUND OF THE INVENTION
Hydrogen storage materials or media (HSMs) are a class of chemical compounds containing hydrogen in a chemically or physically bound form. There is a particular current interest in HSMs for hydrogen storage applications and in particular, for hydrogen-powered vehicles for use in a ‘hydrogen economy’. This use requires an on-board source of hydrogen fuel. Hydrogen storage for transportation must operate within minimum volume and weight specifications, supply enough hydrogen for sufficient distance, charge/recharge near room temperature, and provide hydrogen at rates fast enough for fuel cell locomotion of automotive vehicles. Therefore, in order to create a useful on-board source of hydrogen fuel, an efficient method of storing the hydrogen is required.
Despite optimism over the last three decades, a hydrogen economy remains a utopian vision. The US Department of Energy (DOE) Basic Science group recently summarized the fundamental scientific challenges that must be met before a hydrogen economy becomes viable. In Basic Research Needs For The Hydrogen Ecomony, US DOE Report, May 2003, the following design criteria were identified for a viable HSM:

    
    
        (i) High hydrogen storage capacity (min 6.5 wt % H).
        (ii) Low H2 generation temperature (Tdec ideally around 60-120° C.).
        (iii) Favorable kinetics for H2 adsorption/desorption.
        (iv) Low cost.
        (v) Low toxicity and low hazards.
    
    


Virtually all HSMs used in prior art technologies have been known for several decades, and none of them meet all five of the criteria listed above. For example, a number of alloys such as FeTi, Mg2Ni and LaNi5 satisfy criteria (2)-(5) but fail on criterion 1, containing only a few wt % hydrogen when fully loaded. Li3BeH7 reversibly stores 8.7% hydrogen by weight, but is highly toxic, thereby failing on criterion (5). Materials such as LiBH4 and NaBH4 react rapidly with water (hydrolysis) to release large amounts of hydrogen, but this process is chemically irreversible. Many other materials satisfy criteria (1), (2), (4) and (5), but not criterion (3).
SUMMARY OF INVENTION
In one aspect, this invention provides a reversible hydrogen storage material including on a liquid metal alloy that is a solid in its charged state and a liquid in its discharged state.
In another aspect, the invention provides a method of making a hydrogen storage material comprising a metal alloy that in its discharged state is in a liquid phase, including the step of hydrogenating a ternary alloy.


DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a hydrogen storage material that is a solid in its charged state, but a liquid when discharged. The liquid nature of the discharged material overcomes some of the problems described above, and facilitates the uptake of hydrogen to return the material to its fully hydrogenated state.
In one embodiment of this invention, the discharged state is an alloy of gallium and an alkali metal. Gallium has a melting point just above room temperature (29.8° C.), and such alloys are liquids under normal conditions. Gallium metal can be mixed with an alkali metal to produce a liquid material of the form M3Ga.
In another embodiment of this invention, the new liquid alloy Li3Ga can be used as a HSM. This can be hydrogenated to form the solid compound Li3GaH6, which has a hydrogen content of 6.3 wt % hydrogen, very close to criterion 1 stated above. Another suitable HSM according to the invention is Na3GaH6, which will release H2 around 100° C.
The liquid nature of the depleted Li3Ga renders it mobile and significantly enhances its rehydrogenation to Li3GaH6 and the process circumvents the poor kinetic uptake of H2 associated with many HSMs which are solid in the discharged state (q.v.).
In other embodiments of the invention, the liquid alloy may be Na3Ga (corresponding to Na3GaH6 with 4.1 wt % hydrogen), or a mixed ternary alloy such as NaMgGa (corresponding to NaMgGaH6 with 4.9 wt % hydrogen). For these HSMs, the lower hydrogen content may be compensated by superior hydrogenation and dehydrogenation characteristics.
Materials of the type M3GaH6 may be prepared using the following two methods. The first of these involves direct synthesis under a hydrogen atmosphere, as is the case for the aluminum analogue Na3AlH6 as set out in Eq. 1.




This method is applicable to both ternary products such as Li3GaH6 and quaternary ones such as NaMgH6, as described above.
Alternatively, a two-step method may be used. In the first step a crown ether complex of [GaH6]3− is prepared; this cation is then exchanged for a naked alkali metal counter-ion in the second step, as exemplified in Eqs 2-5, using a large anion to sequester the complex crown ether cation.




It will be apparent to those skilled in the art that materials, such as the tetrahydride LiGaH4, having too fast a decomposition at ambient temperatures[1] are too unstable for use as HSMs according to the invention.

## Claims

1. A reversible hydrogen storage material comprising a metal alloy that is a solid in its charged state and a liquid in its discharged state, wherein the hydrogen storage material is NaMgGaH6.

2. A hydrogen storage material according to claim 1, wherein the hydrogen storage material in its discharged state is a liquid at room temperature.

3. A method of making a hydrogen storage material comprising a metal alloy that in its discharged state is in a liquid phase, comprising the step of hydrogenating a ternary alloy, wherein the ternary alloy is NaMgGa and the material is NaMgGaH6.

4. A method of making a hydrogen storage material according to claim 3 wherein the material in its charged state is in a solid phase.