Note: Descriptions are shown in the official language in which they were submitted.
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, 5 Ca-Mm-Ni BASED HYDROGEhI STORAGE ALLOYS
Field of the invention
The present invention relates to new Ca-Mm-Ni based alloys of the ABS type.
The invention also relates to a process for preparing these new alloys and to
the use of such new alloys for hydrogen storage.
Back~,round of the invention
The use of hydrogen gas as a fuel for PEM fuel cells has received considerable
attention in recent years in view of the fact that PEM fuel cells using pure
hydrogen can provide high efficiency and ultra clean power. Unfortunately,
the widespread use of hydrogen energy is limited by economic and
technological barriers. One of the important barriers is the lack of cost
effective, safe hydrogen storage method.
Hydrogen gas is very light. It can be compressed under high pressure and
stored in pressurized vessels. It can also be liquefied and stored in liquid
form.
Hydrogen also reacts with metal or non-metals to form hydrides. Some metal
hydrides are reversible at ambient temperature and pressures. From a safety
point of view, metal hydrides are intrinsically safe since the hydrogen must
be
released from the hydrides by an endothermic process before it can burn or be
oxidized.
The volumetric density of hydrogen storage in metal hydrides is usually high.
The most serious shortcomings of the reversible low temperature metal
hydrides are their low gravimetric storage density and their high cost. For
stationary and some mobile applications, the weight of the hydrogen storage
tank is not a problem. However, the high cost of conventional low temperature
metal hydrides results in too expensive storage devices.
CaNiS intermetallic compound represents a category of low cost hydrogen
storage materials with a maximum storage capacity up to l.9wt.% (see
reference 1). However, little attention has been paid to this system, probably
due to its well-known bad cycling stability (see reference 2). Improvement of
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2
the hydrogen storage properties of CaNiS by substitution of Ca or Ni with
other elements has been tried (see references 3 to 5). Ternary CaxMmt-XNis and
quaternary CaXMm~_xNi;_yCuy alloys have been produced by melt casting and
patented 20 years ago (see reference 6). Substitution of Mm (mishmetal) for
Ca can raise the plateau pressure of CaNis. However, the plateau slope is big
for the as-cast ternary alloys due to segregation. Annealing at elevated
temperaWres (>1000C) can reduce the slope to some extent. The inventor's
previous work show that CaNis and Mm andlor Zn-substituted CaNiS type
alloys with flat plateau can be successfully produced (see reference 7).
Substitution of Ni by Mm and AI in the CaNiS type alloys can improve the
cycling stability as disclosed in US 4,631,170 (see reference 8). However, the
long term cycling stability of the alloys according to this patent is still
not
good enough. Typically more than 20% of the capacity is lost upon 200 times
of hydrogen absorption/desorption cycling.
Further improvement has been achieved by concomitant substihition of Mm
for Ca and Zn and A1 for Ni, as reported by the present inventors of record
(see reference 9). The capacity loss after 500 cycles has proved to be less
than
20% for Cao_BMmo.2Ni4,8Zno.~Alo,, and less than 10% for
Cao~Mmo,3Ni4,8Alo.lZno,l, However, the maximum storage capacity is
significantly reduced by substitution of Zn and Al for Ni.
Summary of the invention
In accordance with the present invention, it- has now been found that
substitution of Si, Ge and some other metalloid elements (also called "semi- _
metals") for Ni in a ternary Ca-Mm-Ni alloy of the ABS type can substantially
improve the long term stability of such an alloy without causing much
reduction of the storage capacity. Essentially, no capacity loss has been
observed after 500 hydrogen absorption and desorption cycles.
Thus, a first object of the present invention is to provide new Ca-Mrn-Ni
based
alloys of the AB$ type, which axe capable of absorbing and desorbing
hydrogen from a gas phase at ambient temperature with a relative flat plateau
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3
pressure and a storage capacity larger than l:2wt.%. These new alloys are of
the formula (I):
(CaxMl-x)c~l-yTY)s (I)
where M is selected from the group consisting of: any mischmetal, any rare
earth metal, and an homogeneous or an inhornogeneous combination of any
of: (i) at least two mischrnetals, (ii) at least two rare earth metals, and
(iii) at
least one mischmetal and at least one rare earth metal.
T is selected from the group consisting of metalloids and an homogeneous or
an inhomogeneous combination of at least two metalloids;
0<x< 1
0<y< 0.5 and
0.8 < t < 1.2.
Another object of the invention is to provide a process for the preparation of
the above mentioned alloys of formula (I), which comprises the following
steps:
a) preparing a powder by milling a mixture of elemental powders
and/or pre-alloyed substances of the elemental ingredients of
the alloy to be~prepared (such as, for example, Ca, Ni, Mm,
CaNi2, CaNis, MmNis and so on) in adequate proportions to
obtain the required alloy; and
b) annealing and/or sintering the so prepared powder at elevated
temperatures in a crucible for a short period of time in an inert
or reactive atmosphere.
In use, step a) may consist of a ball milling or of a mechanical alloying and
can be carried out at room temperature or at high temperatures with or without
anti-sticking agents.
d
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Step b) is essential to the above process. This step must actually be carried
out
to achieve high reversible capacity and a flat plateau. In use, the annealing
can
be carried in a crucible made of stainless steel at a temperature higher than
600°C but not higher than 1100°C.
Alternatively, the new compounds according to the invention can be produced
by conventional melt casting methods ar powder sintering methods.
The compounds according to the invention are useful for hydrogen storage in a
gaseous form and such is a further object of the invention.
DETAILED DESCRIPTION OF THE INVENTION
As aforesaid, the invention is directed to new alloys of the ABs type, which
are of the formula:
(CaXMi-X)t~l-yTy)s
M can be mischmetal, or any rare earth metal. M can also be an homogeneous
or an inhomogeneous combination of any of: (i) at least two mischmetals, (ii)
at least two rare earth metals, and (iii) at least one mischmetal and at least
one
rare earth metal. In this respect, M is selected from the group consisting of
any mischmetal, any rare earth metal, and an homogeneous or an
inhomogeneous combination of any of: (i) at least two mischmetals, (ii) at
least two rare earth metals, and (iii) at least one mischmetal and at least
one
rare earth metal
T is selected from the group consisting of: metalloids, and an homogeneous or
an inhomogeneous combination of at least two metalloids;
0<x< 1 (x$0); preferably 0.4<x<l;
0< y < 0.5 (y$0), preferably 0<y<0.3; and
0.8 < t < 1.2.
A suitable mischmetal is one of the composition 51 wt% Ce, 26.4 wt% La,
16.4 wt% Nd and 5.3 wt%Pr. A further suitable mischmetal is one of the
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5 composition 62 wt% La, 21 wt°f°Ce; 3 wt% Pr, and 14 wt% Nd.
Many further
compositions are suitable, and differ in the properties of the constituent
elements La, Ce, Pr, and Nd.
Examples of rare earth metals are Y, La, Ce, Pr, Nd, Sm, Dy, Gd, Ho, Th, U,
Pm, Tb, Er, and Lu. Preferably, the rare earth metal is any of La, Ce, Pr, Nd,
Dy, Th, and Y.
Suitable metalloids include Si, Ge, and Ga.
As aforesaid, the invention is actLially based on the discovery that
significant
further improvements have been achieved by substituting Si, Ge and/or other
metalloids for Ni in the above mentioned Ca-Mm-Ni alloy of the ABS type
(see the definition of T in the formula given hereinabove). This substitution
has significant effect of improving the long-term stability while keeping
predominantly the ABS structure and hydrogen storage capacity according to
the invention that are particularly useful.
The new Ca-Mm-Ni alloys of the AB5 type according to the invention with
improved properties can be made by mechanical alloying of elemental
powders (such as Ca, Mm, Nis) and/or mixW res of intermetallic compounds
(such as CaNis, MmNis) corresponding to the required composition, followed
by an thermal annealing treatment at temperatures higher than 600°C for
short
period of time, typically at ,.1000°C or slightly higher for O.Sh-lh in
a steel
crucible. Annealing at temperatures lower than 600C does not improve the
hydrogen storage properties very much.
The invention and its advantages will be better understood upon reading the
following description made with reference to the accompanying drawings.
BRIEF DESCRIPTION OF TI3E DRAWINGS
Fig. 1 is a curve giving the hydrogen storage capacity of an alloy of formula
lCa0.b4 Mm0.36~1.1 Nis as a function of the pressure after 3 cycles and 250
cycles;
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Fig. 2 is a curve similar to the one of Fig. l, but with an alloy of formula
(Cao,s4 ~o.3s)a.i Ni4.9 Sia.i;
Fig. 3 is a curve similar to the one of Fig. 1, but with an alloy of formula
(Cao.sa Mmo.s6O.~ Nia.s Sio.2;
Fig. 4a is a X-ray analysis of the alloy of formula (Cao.~,~ Mrno,36)1.1 N~.s
Sio.a
mentioned hereinabove (see Fig. 3);
Fig. 4b is an X-ray analysis of the alloy of formula (Cat~,~4 Mmo,3~;),,~ Nis
mentioned hereinabove (see Fig. 1); and
Fig. 5 is a curve similar to the one of Fig. 1, but with an alloy of fornmla
(Cao.s4 Mmo.36)~.i Ni4.es Geo.is.
EXAMPLE 1 (Preparation of a known compound by the process of the
invention)
(Cao,6aMmo.36)~.~Nis was synthesized by mechanical alloys in a SPEX high
energy ball mill under argon. A MmNis powder (>99%, + 100mesh), Ca
granules (>99.5, ~2mm in size) and Ni powders 099.9%,-325mesh) were
used as starting materials.
After alloying, an isothermal annealing was performed in a tubular furnace
under argon. The mechanically alloyed powder was sealed in a stainless steel
crucible before annealing. The powder was heated to 1050°C at a heating
rate .
of 30°C/min, and held at 1050°C for 1 hour, then cooled down to
room
temperature in the furnace. ~ __
The hydrogen absorptionldesorption properties were measured by using an
automatic Sievert's type apparatus. The annealed powder normally needs mild
activation treatment, such as heated to 200°C under vacuum and then
cooled
down. The activated (Cao.64Mmo.ss)i.i Nis alloy exhibits a relative flat
plateau
and a maximum storage capacity of 1.44 wt.% under 4.0MPa of charging
pressure.
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A hydrogen absorption/desorption cycling experiment Was performed at
30°C
under an absorption pressure of 3.SMPa and a desorption pressure of
0.01 MPa. The absorption and desorption time was 12 minutes respectively.
Under these conditions, the alloys could be fully hydrided and dehydrided.
The hydrogen purity was 99.999%. As shown in Fig. l, the maximum storage
capacity was reduced to 1.23 wt. % after 250 cycles (20% loss). The reduction
of the effective reversible storage capacity is even bigger.
EXAMPLE 2 (Compound according to the invention made by the process of
the invention)
(Cao.64Mmo.s6O.iNia..9Sio.~ was synthesized by mechanical alloying of
elemental Ca, Si and MmNis powder blends. The alloy was annealed in the
same manner as in Example 1. This alloy had a maximum hydrogen storage
capacity of l.4wt.%. The maximum hydrogen storage capacity are slightly
reduced by 8% after 250 cycles as shown in Fig.2 in contrast to the 20% loss
in the (Cap,g4Mmp.3G)1.IN15~
EXAMPLE 3 (Compound according to the invention made by the process of
the 30 invention)
(Cao,~Mmo,36)i.iNia.BiSio.2 was synthesized by mechanical alloying of
elemental Ca, Si and MmNis powder blends. The alloy was annealed in the
same manner as in Example ~1. This alloy had a hydrogen storage capacity of
l.3wt.%. The maximum and reversible hydrogen storage capacities are
slightly improved upon hydrogen absorption and desorption cycling as shown
in Fig.3. __.
X-ray analyses show that Si-substituted alloys have very high resistance to
peak broadening upon cycling as shown in Fig.4a. While the
(Cao,saMmo.36)~.~Nis alloy without Si substitution shown obvious peak
broadening after cycling as shown in Fig. 4b. It was believed that hydrogen
absorption/desorption cycling introduces defects, such as rnicrostrain,
chemical disorders and grain boundaries (reduced grain size), therefore leads
to reduced storage capacity. The peak broadening reflects the defects
introduced during cycling experiments.
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r5
EXAMPLE 4 (Compound according to the invention made by the process of
the invention)
Cao,~ Mmo,4Ni4,8sGeo.is was synthesized by mechanical alloying of elemental
powder blends. The alloy was annealed in the same manner as in Example 1.
This alloy had a maximum hydrogen storage capacity of l.3wt.% in the as-
synthesized state. Substantial improvement in the maximum and reversibly
storage capacity is observed after 500 cycles as shown in Fig.S.
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y
REFERENCES
(1) "A new family of hydrogen storage alloys based on the system
nickel-misch metal-calcium)" by G.D. Sandrock, proc. 12th
intersociety energy conversion Engineering Conference, 12th
IECEC, Am. Nuclear Society, 1 (1977) 951.
(2) "Stability of Rechargeable hydriding alloys during extended
cycling" by P. D. Goodell, J Less-Common Met., 99 (1984) 1.
(3) "Systematic B-metal substitution in CaNiS" by J.O. Jensen and
N.J. Bjerrum: J. of Alloys and Compounds 293-295 (1999) 185
(4) "Hydriding behavior in Ca-Mg-Ni-B" by H. Oesterreicher, K.
Ensslen, A. Kerlin and E. Bucher": Mat. Res. Bull. 1 S (1980)
275.
(5} "Mechanical alloying and hydrogen storage properties of
CaNiS-based alloys", by G. Liang, J. Huot and R. Schulz, J.
Alloys & Compounds, 321 (2001) 146.
(6) "Nickel-misch metal-Calcium alloys for hydrogen storage" by
G.D. Sandrock, US patent Nos. 4,096,639 and 4,161,402.
(7) "Synthesis ofHnanocrystalline CaNiS-based alloys and use for
metal hydride electrode", by G. Liang, S. Ruggeri, C. Lenain,
H. Alamdari; J. Huot, L. Roue and R. Schulz", J. Metastable
and Nanocrystalline Materials-11 (2001~__7_l. _
(8) "Calcium-Nickel-misch metal-Aluminium quaternary alloy for
hydrogen storage", by K. Ohnishi, T. Ogawa, US patent,
46311?0.
(9) "Synthesis of low cost metal hydrides by mechanical alloying"
by G. Liang and R. Schulz, Report for the CRADA project
(CR-99-004),2001.
