Reversible hydrogen storage composition

Details Reversible hydrogen storage composition Reversible hydrogen storage composition

2001-03-30

2004-05-11

King, Roy (Department: 1742)

Alloys or metallic compositions

Aluminum base

C423S644000, C423S646000, C420S900000, C420S552000, C420S542000

Reexamination Certificate

active

06733725

ABSTRACT:

TECHNICAL FIELD
This invention relates to a hydrogen storage composition and to a method of supplying hydrogen.
BACKGROUND ART
Amongst the known metal hydrides, only a few exhibit properties suitable for reversible hydrogen storage. Reversibility of hydrogen sorption by a metal hydride means the capability to absorb and desorb hydrogen under practical conditions of hydrogen pressure and temperature. Most hydrides are either too stable for hydrogenation cycling so that absorption is easy but desorption of hydrogen requires very high temperatures; or too unstable so that desorption occurs readily, but absorption requires extremely high hydrogen pressure.
Stable metal hydrides include such compounds as LiH, which melts at the temperature of 680° C. but decomposes at an even higher temperature of 720° C., TiH
2
, CaH
2
and ZrH
2
, which have to be heated up to 650° C., 600° C. and 880° C., respectively, in order to release hydrogen. Re-hydrogenation of these hydrides is, however, easy and they absorb hydrogen readily even under low hydrogen pressure.
From the group of unstable metal hydrides, AlH
3
and LiAlH
4
are the most characteristic examples, having such a high equilibrium pressure of hydrogen that gaseous hydrogenation is in practice impossible. Only chemical reactions are effective for the formation of these unstable hydrides. On the other hand, both AlH
3
and LiAlH
4
desorb hydrogen readily at temperatures between 100° C. and 130° C. and with good kinetics.
So far, the only materials which exhibit practical, reversible properties, i.e., hydrogenation/dehydrogenation at ambient conditions, for example, LaNi
5
, FeTi or Ti—V, have a hydrogen capacity of less than 2 wt. %, which is too low for practical applications.
DISCLOSURE OF THE INVENTION
The invention provides hydrogen storage compositions capable of reversible operation at moderately elevated temperatures of 80-180° C., typically 100-150° C., and having a hydrogen capacity exceeding 2 wt. %.
In accordance with one aspect of the invention there is provided a reversible hydrogen storage composition having a hydrogen capacity exceeding 2 wt. % and capable of reversible operation at temperatures of 80-180° C. comprising i) an unstable AlH
3
-based complex hydride alloyed by ball milling with: ii) at least one member selected from the group consisting of: a) an element that does not readily form a hydride in a solid-state form, b) a metal which forms a stable metal hydride, c) a hydride of a metal of b), and d) an unstable AlH
3
hydride complex different from said complex i), said unstable AlH
3
-based complex hydrides i) and ii)d) liberating hydrogen readily at temperatures of 100 to 150° C.
In accordance with another aspect of the invention there is provided a composition of the invention in a dehydrogenated state.
In accordance with yet another aspect of the invention there is provided a method of supplying hydrogen comprising liberating hydrogen from a composition of the invention at a temperature of at least 80° C. with formation of a dehydrogenated form of the composition, removing the liberated hydrogen, and absorbing hydrogen in the dehydrogenated form to regenerate the AlH
3
-based complex hydride as a future source of hydrogen.
In this specification reference to a “dehydrogenated form” or “dehydrogenated state” refers to a form or state of the hydrogen storage composition of the invention resulting from liberation of hydrogen from the composition. It is not intended to indicate that complete dehydrogenation has necessarily occurred, and contemplates both a complete dehydrogenation and a partial dehydrogenation resulting from liberation of at least part of the hydrogen content of the composition.
The reference to an AlH
3
-based complex hydride refers to those complex metal hydrides such as NaAlH
4
or LiAlH
4
which liberate hydrogen readily at moderate temperatures of the order of 100 to 150° C., forming a dehydrogenated form or state from which the hydride form can not be regenerated with hydrogen gas, or which can only be regenerated under extreme conditions impractical in a reversible hydrogen storage composition. These AlH
3
-based complex hydrides are sometimes referred to as being “unstable” in view of their ease in liberating hydrogen and the difficulty in regeneration from the dehydrogenated form.
DETAILED DISCLOSURE OF THE INVENTION
The invention is based on the discovery, that properties of these so-called unstable metal hydrides, which decompose easily but are very difficult to re-hydrogenate, can be altered in such a way that the required re-hydrogenation conditions are much more favourable and the hydride can be regenerated with hydrogen gas in a practical operation.
The alteration of the unstable metal hydride may be achieved by changing the chemical composition of the hydride, accompanied by mechanical grinding.
More especially the invention is concerned with hydrides based on AlH
3
-complex. AlH
3
is very unstable and decomposes spontaneously at temperatures above 100° C. Normally, AlH
3
can not be rehydrogenated, even at extremely high hydrogen pressures, after hydrogen has been liberated from it. The same applies to other hydrides based on AlH
3
complex, for example, LiAlH
4
, NaAlH
4
, Mg(AlH
4
)
2
and Ca(AlH
4
)
2
. These hydrides offer very high hydrogen capacities, typically up to 7-wt. %, and desorb hydrogen easily at temperatures between 100° C. and 180° C., but normally can not be rehydrogenated at hydrogen pressures lower than 100 atmospheres.
In this invention, properties of such unstable hydrides are changed by incorporating in them other elements or hydrides, typically the incorporation may be by alloying the components together by, for example, mechanical grinding or ball milling.
A large variety of unstable AlH
3
-based hydrides have been investigated with different alloying components, and such alloying is found to produce dramatic change in the hydrogenation properties of the AlH
3
-based hydrides. As a consequence, the hydrides become reversible for practical applications because rehydrogenation can be performed at much lower hydrogen pressures.
Typical AlH
3
-based complex hydrides employed in the invention may be represented by formula (I):
M
x
(AlH
3
)
y
H
z
  (I)
wherein M is a metal; x is an integer of 1 to 3, y is an integer of 1 or 2, and z is equal to x or 2x. Preferred examples of M are Li, Na, Be, Mg and Ca, and preferably x is 1 or 3.
Suitable AlH
3
-based complex hydrides for use in the invention include LiAlH
4
, NaAlH
4
, Mg(AlH
4
)
2
, Be(AlH
4
)
2
, Zr(AlH
4
)
2
, Ca(AlH
4
)
2
, Li
3
AlH
6
and Na
3
AlH
6
all of which change their hydrogen sorption properties when mechanically ground or ball milled in the presence of at least one member selected from the following Groups:
1. elements that do not form hydrides in a solid-state form under normal conditions, for example, metalloids such as B, C, Si, P and S, and metals such as Cr, Mn, Fe, Co, Ni, Cu, Mo, Zn, Ga, In and Sn;
2. elements which form relatively stable metal hydrides, such as Be, Mg, Ca, Ti, V, Y, Zr and La;
3. hydrides of the elements from Group 2 above such as BeH
2
, MgH
2
, CaH
2
, TiH
2
, VH
2
, YH
2
, ZrH
2
and LaH
2
;
4. other hydrides based on the AlH
3
-complex.
These additions, alone or in mixtures, are able to change the sorption properties of the AlH
3
-based complex hydrides. The mechanism of the change is not fully understood, but it is probable that different mechanisms are involved with the different classes of additive.
The probable mechanisms of altering hydrogenation properties of AlH
3
based hydrides are as follows:
i) interstitial alloying of the AlH
3
-based hydride.
This mechanism is most probable in the case of metalloids as, for example, boron and carbon.
ii) substitutional alloying accompanied by catalysis.
This mechanism is expected to apply to most metal additions from group 2 elements.
iii) synergetic effect of hydrogen sorption in mixtures of hydrides.
This mechanism was found in mixtures of AlH
3
-based hydrides with hydrides of groups 3 and 4 above a

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