The addition of metals or alloys whose hydrides have a high dissociation pressure allows a considerable increase in the hydrogenation rate of magnesium. The influence of temperature and hydrogen pressure on the reaction rate were studied. Results concerning the hydriding of magnesium rich alloys such as Mg2Ca, La2Mg17 and CeMg12 are presented. The hydriding mechanism of La2Mg17 and CeMg12 alloys is given

Darriet, B.

Hagenmuller, P.

Pezat, M.

English

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NASA TECHNICAL MEMORANDUM
	
NASA TM-75723
USE OF REVERSIBLE HYDRIDES FOR HYDROGEN STORAGE
B. Darriet, M. Pezat, and P. Hagenmuller
(NASA-TM-75723) USE OF REVERSIBLE HYDRIDES
FOR HYDROGEN STORAGE (National Aeronautics
and Space Administration) 18 p
HC; A02/MF A01	 CSCL 10C
NBI-27617
Unclas
G3/44 30086
Translation of "Utilisation d'hydrurps
reversibles pour le stockage de 11hydrogene".
In: Seminar on Hydrogen as an Energy Vector:
Its Production, Use and Transportation.
Brussels, Belgium, October 3-4, 1978.
Proceedings. Luxembourg, Commission of the
European Communities, 1973, pp 392 -406
R J{
NpJ^1
!'?ATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, DC 20546
	 DECE BER 1980
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USE OF REVERSIBLE HYDRIDES FOR HYDROGEN STORAGE*
B. Darriet $ M. Pezat and P. Hagenmuller#*
1. INTRODUCTION
92*OOZ3
For a long time It has been known that many metals eact with
hydrogen in order to form binary hydrides. Out of all the known
binary hydrides, only magnesium hydride has the particular char-
acteristics required for utilization. All of the other binary
hydrides can be eliminated Vor various considerations, either because
they are too expensive or they havo the wrong characteristics for
use as a hydrogen storage agent.
The absorption of hydrogen by tntolniiotallio ev)ilipounds rosults
in the formation of ternary hydrides and this was studied in a less
systematic way. The best known ternary hydrides and the most inter-
esting ones are the following:
- LaNi 5 H 6 and the derived phases
	
(1)
- FeTIH 2 and the derived phases
	
(2)
*Seminar on Hydrogen as an Energy Vector: Its Production,
Ilse and Transportation. Brussels, October 3-4. 1978.
First results of projects funded by the 1! ,^iropean communities.
Directorate-General for Research, Science and Education, 1978.EUR 605 DE/EN/FR/IT
**Solid Chemistry Laboratory of the CNRS, Bordeaux I Universtty.
351 tours de la Lib6ration, 33405 TALENCE, CEDER, France.
***Nwiibers in margin indicate pagination of foreign text.
1
- M92NIH4 and the derived phases
They are characterized by a small mass capacity. In addi-
tion, the high cost of LaNi 5H6 and the slow formation rate of
PeTiH2
 means that they are less interesting than M9 2NiH4 , and
especially more inter ,:-,sting than MgH2.
This compound has attracted special attention because it has
a very high mass capacity, a very low cost
	 and thw magnesium
is available in large quantities in nature.
The question which poses itself is to find and then s _lect
hydrides which are best suited for storing hydrogen. Various cri-
teria have to be taken into consideration for this, and their
importance varies depending on the use to which the hydrides are put;
In order of priority, we will consider the following essential
characteristics:
I
- Large hydrogen mass capacities,
- rapid hydride formation rate and high dissociation rate,
- moderate dissociation temperature,
- low storage pressure,
reduced cost. /391
Even though it is very difficult to simultaneously improve all
of its properties, in practice, the goal to be reached is to opti-
mize them as much as possible.
In this .study we will consider magnesium and magnesium alloys
with special consideration of two essential criteria, which are a
high hydrogen mass capacity and a very low cost price.
In our first analysis, we were forced to improve the synthesis
conditions for magnesium hydrides.
After this, we studied magnesium alloys in order to satisfy
I 2
in a more satisfactory way a large number of essential criteria
for storing hydrogen. It should be realized that the studies on
this latter point have only started. We believe that research in
this area will result in alloys which can 'bring about better per-
formance.
2. EXPERIMENTAL PROCEDURES
We used commercial products whose concentration varied between
97 and 99%, horning forward to industrial applications. The mag-
nesJ um utilized is a 986 powder product.
The intermetallic compounds or alloys used are prepared in a
laboratory.
a) Production of alloys
In the case of the alloys, which do not have elements with
a high vapor pressure, the preparation is made by fusion°in an are
furnace.
Fusion in a sealed iwl4ybdenum tube in an argon atmosphere is
usually done with magnesium alloys.
- by fusion in an are furnace:
After mixing the elements in appropriate proportions in a
glove box in an argon atmosphere, pellets are made under 5 T/cm2:
After fusion in the arc furnace in argon, the product is then
possibly reheated.	 /395
- by fusion in a sealed molybdenum tube:
The magnesium alloys cannot be prepared by the previous method
because of the hi ,h vapor pressure of the magnesium.
The procedure is identical with the preceding one as far as
the preparation of the melting mixtures is concerned. The procedure
3
rc	 ^
only differs in the fusion locality, which iP performed in a sealed
molybdenum tube in an argon atmosphere. In all these cases, fusion
is followed by reheating over several hours at a temperature which
is suitable for each alloy.
b) Charac,^ fer nation
After each preparation is made, there is a radio-crystallo-
graphic check which is carried out in order to identify the phRse
or the phases which are present.
It is possible that a precise determination of the cr ystallo-
graphic parameters will be made by adding an internal standard in
the sample.
c) Activation of alloys used as adjuvants
As we will see later on, certain alloys such as LaNi S for
example, are mixed in the form of powder with magnesium in order to
improve synthesis conditions of the Latter.
The preparation of these alloys in an arc furnace in general,
leads to a compact metallic block. In order to reduce this block
to the powder state, it is necessary to carry out several absorp-
tion-desorption cycles of the hydrogen, which alloys the alloy to
be made fragile and to obtain a very fine powder.
In the case of LaNi 5 , this operation is carried out at ambient
temperature under a pressure of 150 bars.
d) Determination of the absorption and desorption rates
The amounts.-of absorbed or desorbed hydrogen as a function of
time are determined by a manometric method.
`..	 4
3. EXPERIMENTAL RESULTS
	
ZIL
Table I shows the main characteristics of magnesium hydride.
TABLE I
molar mass	 2b.33b
density at 250 C	 1.45
mass capacity for H 2	7.65%
formation heat
	 -17.8 kcal/mole H2
decomposition temperature
at 1 atmosphere of H 2____, 	 2870C
Out of all the known binary hydrides, MgH2 is the most pro-
mising because of its high mass capacity for hydrogen (7. 65% ) and
its very low cost.
The essential factor which is not favorable for xgH2 is the
difficulty associated with its directs preparation by means of the
t=ction of hydrogen on magnesium.
For strong hydrogens pressures (on the outer of 150 bar) and
at a temperature close to 500°C, the reaction rate is still very
slow.
Considering the properties of MgH2 . we found it interesting to
improve its synthesis conditions. For this purpose, we added an
adjuvant to the magnesium (metal or alloy) which easily reacts with
hydrogen but whose hydrides are thermally less stable than MgH2.
a) Mg adjuvant system
Hydride formation
We successively examined the influence on the absorption rate
of hydrogen by magnesium of each of the following parameters:
5	 ,^
the characteristics of the adjuvant
. the proportion of adjuvant
the temperature
. the hydrogen pressure.
In all of these cases the amounts of hydrogen which were
absorbed are expressed in percentages of hydrogen fixed by the
magnesium.	 This corresponds to the proportion of hydrized mag- /397
nesiur; with respect to the total mass of magnesium which Pan
react.
Figure 1 shows the absorption of the hydrogen as a function
of time for various mixtures (Mg-adjuvant) at 345 0C at 30 bars for
a weighted percentage of 80% of magnesium and 20% adjuvant. It
shows that in all cases, the addition of the adjuvant increases the
hydride formation rate of the magnesium. The system Mg-LaNi 5 seems
to be the one with the highest performance.
One could believe that the presence of the adjuvant catalyzes
the reaction by p roducing atomic hydrogen in situ which is much
more reactive with respect to magnesium.
For the same structural type of adjuvant (that of LaNi 5 ), we
wanted to see whether the relative stability and the stoichiometry
of the corresponding hydride could influence the hydride formation
rate of the magnesium. The experimental results obtained for the
various alloys suoh as LaCo,, CeCo 5 , LaNi 5 and Sm'Ni 5 show that they
have a behavior which is comparable, no matter what: the correspond-
ing hydride characteristics are. The hydride formation rates are
or even slightly less than in the case of SmNa.,, which gives the
least stable hydride and the hydride with the smallest hydrogen con-
centration.
Considering the relatively high cost of these various
adjuvants, we considered a s ystem in„ which one uses the
6
i
igixed metal Ni 5 (MNi 5 ) as an adjuvant. The results obtained show
that the action of MNi 5 ola the hydride formation rate is similar.
The amount of stored hydrogen is of the same order of magnitude as
that which is fixed by LaNi5. Thus, it follows that the performances
which were obtained with an alloy such as LaNi 5 are essentially
the same if one uses MNi,, but the use of the latter is more advan-
tageous because it has a coat
	 which is substantially below that
of LaNi5.
C
For 345 0 G and for 30 bars, Figure 2 gives the result for
various proportions of LaNi 5 in the mixture Mg-LaNi5.
The amount of absorbed hydrogen increases w1th the amount of
LaNi5 . Also the increase in the percentage of adjuvant brings
about a reduction in the theoretical mass capacity, as shown by
Table Il.
TABLE Il
	 /198
LaNi in the theoretical % H2
 fixed at
	 experimental mass
mixtur^	 mass capa-
	 the end of 1	 capacity at the
city at 25°C hour
	
end of 3 hour
20	 6.4
	 87.2
	 5.62
10	 7.0	 69.1	 4.89
From the economic point of view, and considering these results,
only the costs	 of the various mixtures will allow the select-
ion of the most interesting proportion.
The curves given in Figure 3 show that the hydride formation
rate of the magnesium increases with temperature. For a given time,
this increase varies in a linear fashion as a function of temperature.
The increase in the hydrogen pressure influences primarily the
hydride formation rate at the beginning of the reaction (Figure 4).
At the end of one hour, the ;influence of this factor seems to be
7
negligible for pressures between 10 and 60 bar.
- Desorption:
The curves of dissociation of MgH 2 (Figure 5) show that the
desorption rate at 2 Lars depends strongly on temperature. By
judiciously selecting this parameter, one can desorb the same
amount of hydrogen for a time which varies at a ratio of between
1 to 10.
- pressure-composition isotherms:
Because of the improvement in the synthesis condition MgH2,
smaller pressures and lower temperatures, it is possible to draw
pressure-composition isotherms for temperatures between 280°C and
:'^60 0 C. The results obtained in Figure 6 show a hysteresis pheno-
menon.
b) Magnesium alloys
We wanted to keep a large mass capacity and, therefore, decided
to find alloys which are rich in magnesium and which could react
with hy',drogen, and whose hydrides would be easier to dissociate
than magnesium hydrides.	 a9_9
- M92 Ca
In the system Mg-Ca, the only defined component found is M92 Ca.
The theoretical mass capacity of the hydride M9 2CaH6 which can be
formed is 6.3%.
M92 Ca is prepared by fusion in an argon atmosphere in a sealed
molybdenum tube. The sto ichiometric mixture in the proportions of
the two Mg atoms and one atom of Ca is heated to 700°C aor about
15 hours. This thermal treatment is followed by a reheating at
^•	
8
600°C for two days,
The curves of Figure 7 show that the hyd."ide formation rate
of M92Ca is relatively slow. At 400 0C under 50 bars, the percent-
age of fixed hydrogen is 46 % at the end of 40 hour, of reaction.
By adding LaNi 5 to Mg,Ca (50% 14g 2Ca - 10% LaNi 5 ), this in-
creases substantially the reaction rate out does not result in a
high performance of the system ( 62% of fixed hydrogen at the end of
40 hours).
- La2 Mg, 7
The structure of La2Mg l7 derived directly from that of LaNi,
by replacement of a lanthanum atom by two magnesium atoms. The
structure of LaNi 5 11 a host structure for t^,e hydrogen, and we,
therefore, believe that La2Mgl7. could be of interest for storing
hydrogen.
The alloy La 2Mgl7 
is obtained by fusion of a mixture at
appropriate proportions of the different constituents. The pre-
paration is prepared in a sealed mol ybdenum; crucible in a dry argon
atmosphere and is deoxygenized. After heating to 700 0 0 for one
hour, the alloy is reheated for 10 hours at 500°C. Either the
alloys are used directly in the form of metallic blocks, or are
reduced to the powder state which could possibly be in tablet form..
The high magnesium content of this alloy allows one to predict
a high theoretical mass ca.pc, ^ity: 5.5% for the formula La2Mgl7H40.
The first results show that at 350°C and at a pressure of 30
bars, the percentage of absorbed hydrogen is on the order of 100%
at the end of 10 hours. 	 In addition, the hydride formation rate /400
i;: very rapid: 75% of absorbed hydrogen at the end of five minutes.
9
A more detailed study shows that in the study temperature and
pressure range, the action of hydrogen can La 2Mgl7 occurs in two
stages:
- in the first cycle, the hydride formation results 3,n
irreversible decomposition of the alloy and formation of hydrides
corresponding to each of the elementsi
La2Mgl7	+	 24H2	 41	 2LaH3	 +	 17MgH2
starting at the second cycle, the reaction is carried out in
a reversible manner according to the foll owing reaction diagram:
2LaH	 + 17MgH2	2LaH2.	 + 17M9 +	 H2.
2
Desorption has characteristics which are very close to those
observed for the Mg-adjuvant mixture.
- CeMgl2
Just like the compound La2Mg i7 , the compound CeM912 is derived
from a structure of the type La.Ni 5 by replacement of an atom of Ce
by two magnesium atoms.
The procedure for preparing the CeMgl2 is similar to that
for La 2M917'
The first results obtained with CeMg12 show that they are com-
parable with those for La2Mgl7.
The hydride formation mechanism occurs in two stages as for
the La2Mg17.
We compared the behavior of a mixture with the proportions
(Ce + 12Mg) to the behavior of the alloy CeMgl2.
10
Figure 9 shows how interesting the CeMg12 is for the mixture
(Ca + 12Mg). In the two cases, we can see the catalytic role
which is played by the cerium. We can explain the higher hydride
formation rate of the Mg by not using CeMg12 due to the fact that
hydride formation results in a more homogeneous distribution of the
various elements and in a much finer particle formation.
	 An
From these first results which are very encouraging, we will.
carry out a systematic study of this compound as a function of
various parameters (pressure, temperature, etc.) which could
influence the condition for preparation and utilization of this
hydride.
4. CONCLUSIONS
The results obtained show that the hydride formation rate of
magnesium is considerably improved by the presence of another metal
or alloy which forms a hydride which is thermodynamically less stable
than the MgH26
We made a detailed study of the influences of the various
parameters on the synthesis conditions and on the hydride forma-
tion ra y;;; of magnesium for various Mg-adjuvant systems. The study
of each of theso parameters allowed us to show that the system
Mg-LaNi 5 . has the highost performance, and we were able to formulate
the optimum utilization conditions.
From an economic point of view, it seems to be of interest to
substitute the misehmetal Ni s for the LaNi 5
 in the systems.
We became interested in ma
magnesium for which encouraging
The study of La 2Mgl ?
 which
pound has characteristics which
hydrogen. The results obtained
gnesium alloys which were rich in
results were obtained.
was carried out shows that the com-
would be of interest for storing
allowed us to show that the first
11
nf
hydride formation leads to the formation of a hydride mixture
(LGH3 + MgH2).
Starting with the second cycle, the system is reversible and
has characteristics which are of interest for storing hydrogen.
In the case of CeM9 12 we were able to show that the perfor-
mances obtained with this compound were greater than those obtained
with the mixture Ce + 12 Mg.
!	 At the present time we are carrying out a study about alloys
`	 of the same family as La2Mg17 and 
ner"912 in order to increase the
number of reversible hydrides which could be used for staring
hydrogen.	 /402
REFERENCES
(1) J. H. N. VAN VUCHT ;, F. A. KUIJPEHS and H. 0. A. M. BRUNING,
Philips Res. Repts. 25, 133-14o, 1970.
(2) J. J. REILLY and R. H. WISWALL, Jr., Snorg. Chem., 13 (11),
21&, 1974.
(3) J. J, REILLY and R. H. WISWALL, Jr., Inorg. Chem., 7 (11)
2254, 1968.
12
0/0 h
lot
5( L21
PO
All	
10%
%lif
L# Ni,.,
V
Msr?Nt
^49
Is 30	 'S t (min7
Figure 1.	 Hydride, fox-mat ion at 345 0C and at a pressure of
30 bars of.. various magneslum-adjuvant mixtures
(40% m 
t	 I	 t
	
 , - - , , I-	 ^, ­
014	 10	 45	 1 mill
Figure L). Hydride formation at 3 11 1)"o and at a i)ressure of
30 bars ,,r the mixt.-ures (Me + LaNI 
5 ) for various
IIY
proportiona of LaNi 
5
lit100
400%,
rt
SU
0
	
15	 3n	 45	 t min
Figure 3. Hydride formation at a pressure of 30 bars of the
mixture (Me + I'OaNi	 (10%) at various temperatures 
13
%H
100
5
/11011
Min
Figure 1 . Hydride formation at 3115 1 0 or the mixture (DJg +LaNI 5 ) (10%) at Various hydrogen pressures
% H ik
100-
42.00C
345 -Q,
so.
0	 10	 20	 30	 Ems.
Doeomposition at various temperatures and at aPres sure Or two bars of the hydride form froill
the initial -mixture (MR-LaNi-) (lot).
^ 11
/x+05
ce
n^
i
P680
15
10
5
C
% H
loc
5c
15
0^	 1	 2 H '/Mg
Figure 6. Pressure-composition isotherms of the system
(Mg + LaNi 5 ) - H2
Figure 7. Hydride formation at 400 0 C and at pHH = 50 bar
of M9 2Ca and of 90% M9 2Ga + 10 Larli5
sH
'11150f I11
10
%H
too
/1 06
.-IN.
IS	 30	 45	 t (minute)
Hydride formation of La 2Mgl7 at 325°C at a
pressure of 30 bars
1
so
Figure 8.
O
	 30...-...,..,.1_...^:,. ..	
r>-1G(1 t minS
Figure 9. Hydride formation at 325 0C and at p., = 30 bar
a) of CeMgrz (second cycle)
b) of a mixture of Ce + 12 Mg


Use of reversible hydrides for hydrogen storage

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