Note: Descriptions are shown in the official language in which they were submitted.
~C-2889/CAN
The present invention is direc-ted to a method
for storing hydrogen by absorption in nickel~mischmetal-
calcium alloys.
The use of hydrogen gas as a fuel has received
considerable attention during recent years because hydrogen
can be generated by a variety of methods that do not rely
on fossil fuels, te.g., solar energy, nuclear energy, and
water power). One of the principal problems confronting
wide acceptance of ~ydrogen as a fuel is related to storage.
At present, hydrogen is commonly stored under relatively
high pressure, e.g.~ 136 atmospheres, in steel storage
cylinders. This type of storage is adequate for many
applications; however, due to weight and bulk requirements,
such high-pressure cylinders cannot be readily adapted to
the requirements of operational units such as ~ehicles.
Furthermore, in many instances, the required high pressures
are considered unsafe.
In order to circumvent the problems attending
conventionally used storage methods, considerable attention
has been directed recently to the storage of hydrogen as a
hydride. Compounds of the type AB5 and commonly referred
to as a CaCu5 type of structure have received considerable
attention. The compounds have a hexagonal crystal structure
and are capable of absorbing hydrogen to a volume density
of almost twice that found in liquid hydrogen, roughly
6X1022 atoms/cm3.
In my co-pending Canadian application No.
288,556 filed October 12, 1977, it is shown that CaNi5
offers the capability for storiny hydrogPn at sub-
atmospheric pressures. This discovery is in direct contrast
,,~
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to the publication by H.H. Van Mal, K.H.J. Buschow, and
A.R. Miedema reported in the Journal of the Less-Common
Metals, Vol. 35, (1974), which showed an adsorption/
desorption plateau of about 15 atmospheres ~or CaNi5.
It is known from U.S. Patent No. 3,825,418 that
Ni5M compounds (where M represents mischmetal) are useful
for hydrogen storage. However, the minimum hydrogen pres-
sure required for sorption is reported therein to be about
41 atmospheres (600 psi) at 25C.
The preparation of a nickel-rare earth (lanthanum)-
calcium compound is shown in U.S. Patent No. 3,883,346.
EIowever, this patent shows that the residual quantity of
calcium is undesirable, limiting this element to 0.4 weight
percent.
The aforedescribed publication and patents are
concerned with compounds or alloys that require relatively
high pressures for absorption and storage of hydrogen.
As a consequence, the requirement remains for relatively
heavy-walled, low-alloy steel containers, albeit not quite
as strong as conventional hydrogen storage cylinders.
It has now been discovered that hydrogen can be
advantageously stored in a nickel-mischmetal-calcium compound
at pressures between about l atmosphere and 15 atmospheres.
Generally speaking, the present invention is
directed to a method for storing hydrogen at pressures
ranging from about l atmosphere to about 15 atmospheres
comprising contacting a hydrogen containing gas with a
granulated Ni5Ml yCay compound, where M represents mischmetal
and y is from about 0.2 to about 0.9.
~ s~
Compounds ranging from Ni5Mo 8CaO 2 to Ni5Mo lCaO 9
provide variable hydrogen gas storage at 25C at pressures
from about 15 atmospheres to about 1 atmosphere respectively
and dependent upon the particular composition of the com-
pound as defined by the equation
P=28.5 exp.(-2.9y)
at a H/M ratio of 0.5 (atomic ratio of the number of
hydrogen atoms to the number of metal atoms), at 25C,
with P being the desorption pressure in atmospheres. The
ratio of nickel to mischmetal plus calcium on an atomic
basis should be from about 4.5 to about 5.5 and preferably
from about 4.8 to about 5.2.
The foregoing shows that more favorable hydrogen
storage conditions, from the standpoint of high pressure
safety, can be provided than available through the use of
a NisM compound which is relatively unstable and for which
a hydrogen dissociation pressure at 25C of about 29
atmospheres was determined experimentally. Conversely,
low pressure storage is available in compounds such as
Ni5Mo lCaO g of the same magnitude as available in a known
Ni5La compound, i.e., about 1.7 atmosphere; however, the
cost per gram of hydrogen stored is significantly lower
in the nickel-mischmetal-calcium compound. To illu~trate,
on the basis of raw material cost alone, a Ni5La compound
costs, at current prices, about $1.49 per gram of hydrogen
stored, whereas a Ni5Mo lCaO g compound costs $0.44 per
gram of hydrogen stored.
Although the compounds of the present invention
are significantly lower in cost than Ni5La compounds, they
are priced substantially higher than iron-titanium compounds
which cost, on a raw materials basis, about $0.20 per gram
of hydrogen stored. However, the eompounds of the present
invention are substantially insuscep-tible to poisoning by
gases such as 2~ CO, CO2, CH4, etc. Contamination by
sueh gases restricts the capaeity of iron-titanium eom-
pounds for hydrogen storage and limits their use to gas
streams eontaining high purity hydrogen.
The eompounds of this invention are generally
prepared on a weight basis since the individual ingredlents
are eombined by melting; and hence, it is convenient to
deseribe the compound in the terminology eommonly used for
alloy preparation.
The compounds or alloys of this invention eon-
tain, in weight percent, from about 4% to about 27% miseh-
metal, from about 2% to about 11~ ealcium, with the balance
essentially nickel. In order to minimi~e the eost of the
raw materials used in the preparation of the alloy, it has been
found expedient to substitute some eopper in place of the
niekel. Up to about 17.3 weight pereent eopper ean be 5ubsti-
tuted for niekel for this purpose. However, substitution of
eopper substantially lowers the hydrogen storage eapaeity
o the alloy so that on the basis of material eost per gram
of hydrogen stored, this substitution is considered to be
more expensive than the nickel~mischmetal-caleium alloys
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as will be shown hereinafter. E~owever, the substitution of
copper in part for nickel can serve to improve the resistance
of the alloy to contamination by gases such as CO, CO2,
N2, etc.
Preferred alloys contain, in weight pereent, from
about 6% to about 15% mischmetal, from about 6% to about
10% calcium, and the balance essentially nickel. A most
preferred alloy contains about 12% mischmetal, about 8%
calcium, and the balance essentially nickel. Such a
preferred alloy offers the lowest raw material cost per
gram of hydrogen storage and a favorable dissociation
pressure ranging from about 6 atmospheres down to about
1.3 atmospheres absolute.
As will be understood by those skilled in the
art, the use of the expression 7'balance es.sentially" does
not exclude the presence of other elements commonly present
as incidental elements, e.g., the deoxidizing and cleansing
aid elements, and impurities normally associated therewith
in small amounts which do not adversely affect the novel
characteristics of the alloys.
The compounds or alloys can be prepared by air
melting or by vacuum melting and casting into ingot molds
as hereinafter described.
Following cooling to room temperature, the ingots
are removed from the molds and crushed to granular form.
A U.S. Standard Mesh Size of about -4 has been found
appropriate for hydriding applications.
The crushed and sieved alloy is introduced to a
suitable, valved pressure vessel for hydriding. Initial
hydriding or activation can be accomplished by evacuating
37
the vessel and then introducing gaseous hydrogen at ambient
temperatures and pressures above about 5 atmospheres, twith
the minimum pressure dependent on composition). The alloy
begins to absorb hydrogen almost immediately and is gener-
ally fully hydrided within about one hour. Once the
nickel-calcium-mischmetal alloy is charged with hydrogen,
the vessel is valved oEf and ready for use as a source of
hydrogen. Subsequent recharging with hydrogen is accom-
plished in time periods significantly less than one hour,
e.g., lQ minutes.
The aforedescribed vessels can be used for any
number of applications, including the provision of a
hydrogen atmosphere to ~ furnace, as a fuel source for an
internal combustion engine, etc.
For the purpose of giving those skilled in the
; art a better understanding of the invention, the following
examples are given:
EXAMPLES
Eight kilogram heats having the compositions shown
in Table I were prepared by induction melting. The alloys
identified as 1 and 2 were melted under vacuum in an alumina
crucible. The alloys identified as 3 through 6 and the
alloys outside of the invention identified as A and B were
all prepared by air melting in a clay-graphite crucible,
(e.g., a number 30 crucible sold under the Trademark
DIXAGRAF and available from Joseph Dixon Crucible Company).
In the air-induction melting practic~, nickel is
melted, mischmetal added, and the calcium plunged below the
surface of the melt. The melt is induction stirred to
~8~
provide thorough mixing of the ingredients and poured into
ingot molds.
A vacuum melting practice consists of melting
nickel under a vacuum of about 10 2 Torr, adding mischmetal,
backfilling the vacuum chamber with argon to a pressure of
about 380 Torr, adding calcium, inductively stirring for
about one minute, and pouring into ingot molds. The melt
temperature should be maintained below about 1500C to sub-
stanitally avoid reduction of the alumina crucible.
Mischmetal is a mixture of rare-earth elements in
metallic form; the rare-earth elements have atomic numbers
between 57 and 71. The commercially produced mischmetal
used to prepare the alloys of this invention was obtained
from the Molybdenum Corporation of America and contained
about 48 to 50% cerium, 32 to 34~ lanthanum, 13 to 14
neodymium, 4 to 5% praseodymium, and about 1.5% other rare-
earth metals. As those skilled in the art will understand
other commercially available grades of mischmetal can be
substituted for preparation of the alloys of ~his invention,
(e.g., a cer~um-free mischmetal).
Eight grams of -10 mesh, +14 mesh granules of
the nickel-mischmetal-calcium alloys were placed in a 15mm
diameter by 35mm high reactor vessel. A vacuum pump was
used to remove air from this chamber to achieve a vacuum of
about 10 2 Torr. The vacuum source was valved off and ultra
high purity hydrogen introduced to the apparatus. For
experimental purposes, a hydrogen pressure of 68 atmospheres
was used to pressurize the apparatus. It was observed that
the specimens began to activate immediately and absorb
lar~e quantities of hydrogen. The specimens ~ere essen-
tially saturated with hydrogen within a time period of
generally about one hour.
Hydrogen desorption pressures were measured at
25C as a function of H/M ratio (atomic ratio of the number r
of hydro~en atoms to the number of metal atoms). E'or each
alloy, a slopin~ dissociation pressure plateau was found
to exist for H/M ratios from about 0.2 to about 0.7.
10Table II shows the results of the dissociation pressure
tests for the alloys identified as 1 through 6, ~ and B.
The dissociation plateau for Alloy A, a nickel-mischmetal
alloy, ranged from 26.0 to 31.5 atmospheres as compared to
dissociation pressures of 13.0 to 15.4 for the nlckel
mischmetal alloy, No. 1, containing about 2 weight percent
calcium. At the other end of the spectrum, Alloy B, a
Ni5Ca compound, had a sub-atmospheric dissociation pressure
of 0.41-0.56 atmospheres, whereas the dissociation pressure
of a nickel-calcium alloy containing about 5%, by weight,
mischmetal was 0.8-1.8 atmosphere. Thus, Alloys 1 through
5 show that levels of dissociation pressure intermediate
to that available in either the excessively high values
available in nickel-mischmetal or the excessively low
values available in nickel-calcium.
Also shown in Table II is an alloy, identified as
No. 6, containing one part of copper substituted for one of
the normal 5 parts of nic~el. Although copper lowers the
dissociation pressure somewhat, it decreases the hydrogen
TABLE I
IDENTIFICATION AND COMPOSITION
OF HYDRIDABLE COMPOVNDS
Analysis in weigh-t percent, balance Ni
Approxima-te Actual Rare
Alloy Value of y Formula of Earth
Identity ~ Cay Compound Ni Metals Ca O N C Al
0.2 5 0.80 0.19 1.8 0.0011 0.0050 (2) 0.032
2 0 5 5 0.49 0-45 4.85 0.0005 0.0075 (2) 0.064
3 0.7 Ni5Mo 29Ca0 68 81-3 11-4 7.53 0.030 0.069 0.023
4 0.8 Ni5Mo 20CaO 78 83.2 7.86 8.86 0.059 0.055 0.022 (2)
0.9 Ni5Mo 10Ca0 89 85-5 4.29 10.4 0.12 0.074 0.024 (2)
6 0.7 4 1 0.30 0.67 ()7.36 0.047 0.056 0.017 (2)
A 0 Ni5M1 0068.0 32.3(2) 0.010 0.006 (2) 0.029
B 1 Ni5CaO 9888.5 (2)11.8 0.038 0.085 0.018 (2)
, ~
(1) Also contains 17.3% Cu.
(2) not analyzed.
8~ ,,~
TABLE II
Dissociation pressures for H/M rations from 0.2 to 0.7 for
alloys based on Ni5 Ml_y Cay where y ranges from 0.2 to 0.9
Alloy Value of y in Dissociation pressure,
Identity Ni5Ml yCay _atmospheres
1 0.2 13.0 - 15.4
2 0.5 3.5 - 12.3
3 0.7 1.4 - 6.2
4 0.8 1.5 - 4.7
0.9 0.8 - 1.8
6 0.7 1.0 - 15.5
A 0 26.0 - 31.5
: B 1 0.41 - 0.56
TABLE III
Raw Material Cost per Gram of Hydrogen Stored
Alloy Approximate Formula Dollars/Gram
Identity of Comvound of Hydro~en
1 N i sM o. a C aO- 2 ~ 37
: 2 Ni5MO.5CaO.~ 0.38
3 Ni5MO.3CaO-7 0~35
4 Ni5M 0.2Ca 0-8 - 35
S Ni5MO.1CaO.g 0.44
6 Ni~Cu1MO.3CaO.67 0.48
A Ni5M 0.43
C Ni5La 1.49
--10--
37
storage capacity. Thus, on the basis of raw material cost
per gram of hydrogen stored, the copper-containing alloy
is somewhat more expensive than copper-free alloys as shown
in Table III.
~ lthough it is realized that the cost of preparing
any one of the alloys of the present invention is dependent
on other factors such as meltinq cost, crushing cost,
activation cost, and resistance to gas impurities, Table III
shows that the nickel-mischmetal-calcium alloys are price
competitive with nickel-mischmetal, and particularly nickel-
lanthanum. Consequently, i~ addition to the advantages of
hydrogen absorption at lower pressures, substantially
improved insusceptibility to contamination and ease of
preparation, the alloys of this invention appear -to offer
a cost advantage over presently existing nickel alloys
useful for hydrogen storage.
Hydrogen can be absorbed and desorbed from the
nickeI-mischmetal-calcium alloy at ambient temperatures of
from about -30C to about 100C, preferably from about
0C to about 80~C. The dissociation pressures referred
to in Table II were measured at 25C.
Although the present invention has been described
; in con~unction with preferred embodiments, it is to be
understood that modifications and variations may be resorted
to without departing from the spirit and scope of the
invention, as those skilled in the art will readily under-
stand. Such modifications and variations are considered to
be within the purview and scope o~ the invention and appended
claims.
