|Title of Invention||
"METAL ALANATES DOPED WITH OXYGEN"
|Abstract||A metal alanate material useful for reversible hydrogen storage as in fuel cell applications includes a metal alanate material that is doped with oxygen. In discussed examples, the metal alanate material is one of an alkali metal alanate or mixed alkali metal-alkaline earth metal alanate. In some examples, the oxygen is doped into the metal alanate from an unstable solid oxide having -&Dgr;Gf° <200 Kcal/mole or from a hydroxide, a carbonate, a nitrate or an oxygen gas mixture. The metal alanate in one example is doped with between 0.5mol% and 30mol% oxygen.|
|Full Text||METAL ALANATES DOPED WITH OXYGEN
BACKGROUND OF THE INVENTION
This invention relates to reversible hydrogen storage material. More particularly, this invention relates to a metal alanate material that is doped with oxygen, thereby allowing increased hydrogen absorption kinetics and storage capacity compared to previously known doped metal alanate materials.
Metal alanates, such as NaAlHU, are generally known as reversible hydrogen storage materials. A metal alanate stores and releases hydrogen, and can be replenished with hydrogen at moderate pressures and temperatures. At approximately 80°C dehydrogenation (i.e., liberation of hydrogen) of the metal alanate is thermodynamically favorable. In a reverse rehydrogenation reaction at 100°-120°C and 60-100 bar, hydrogen is recharged back into the metal alanate. Fuel cell devices, for example, can utilize metal alanates because of these relatively temperate dehydrogenation and hydrogenation conditions.
In applications such as a fuel cell device, greater volumetric hydrogen storage capacity is desirable. In an effort to increase the hydrogen storage capacity of conventional metal alanates, it has been proposed to add dopant amounts of certam transition metals as thermodynamic catalysts. Typically, doping with approximately 2-6 mol% of the transition metal, such as Se, Ti, or Zr, signifîcantly increases the hydrogen absorption and desorption kinetics.
A drawback of using conventional transition metal dopants is the diminishing, or negative, effectiveness of the dopants in amounts over 2mol%. For instance, the hydrogen absorption of NaAlH decreases substantially when the amount of a Se dopant increases from 2.0mol% to 3.3mol%. The limit of effectiveness of a Se dopant is 2.0mol%. Ti has been effectively used as a catalyst ui NaAlHt up to 6mol% concentrations. These higher levels of dopant come at the cost of increased halide content, which fonns NaQ or NaF thus reducing overall capacity. Increasing catalyst content over 4mol% is thus undesirable.
A metal alanate material that provides increased hydrogen storage capacity beyond that which is available from the limited effectiveness of conventional dopants is needed. Mechanical milling of NaAlKU with some oxides such as
and CeOa have been noted in the literaturo with only slight enhancement of kinetics. Utilizing oxides with -AGf°>200 Kcal/mole such as AlaOs and CeO2 does not lead to incorporation of the oxygen into the system and thus limited kinetic activity.
SUMMARY OF THE INVENTION
In general terms, this invention is a metal alanate material used for reversible storage of hydrogen as in fuel cell applications.
In one example, the metal alanate base material is one of an alkali metal alanate or a mixed alkali metal-alkaline earth metal alanate. The base metal alanate material is doped with approximately 0.5%-30% oxygen (on a molecular basis) to thereby enhance the hydrogen storage kinetics and capacity of the material.
In one example, the source of dopant oxygen is a solid oxide. The solid oxide is selected from a group of unstable solid oxides, those with a -AGf° In one example, the solid oxide is doped into the metal alanate using a known ball-milling technique. Alternatively, the oxygen may be introduced to the metal alanate by a gas mixture including oxygen gas and an inert gas.
A metal alanate doped with oxygen allows the dopants, such as Se, to be used in amounts that exceed the previous limitation of effectiveness of 2mol%. Metal alanates doped with oxygen provide an improved reversible hydrogen storage material and exhibit favorable kinetic and thermodynamic characteristics required for use in fuel cell devices, for example.
The various features and advantages of this invention will become apparent to those sMlled in the art from the following detailed description of the currently prefened embodiments. The drawings that accompany the detailed description can be briefly described as follows.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a general schematic view of an automobile having a fuel cell device with a hydrogen storage portion designed according to this invention; and
Figure 2 graphically shows example hydrogenation results using an example metal alanate material designed according to the invention.
DETAILED DESCRBPTION OF THE PREFFERRED EMBODIMENT
Figure l schematically shows an automobile 10 utilizing a fuel cell device 12 for power. In generating power, the fuel cell device 12 requires hydrogen, and therefore an on-board source of storing the hydrogen. A hydrogen storage portion 14 of the fuel cell device 12 includes a metal alanate material that is doped with oxygen.
The base material of the metal alanate material can be an alkali metal alanate, a mixed alkali metal-alkaline earth metal alanate or a transition metal alanate. The alkali metal alanate in one example preferably is NaAlH and the mixed alkali metal-alkaline earth metal alanate preferably is described by the formula:
where M1 is an alkali metal; M2 is an alkaline earth metal; and O Mx1 My2 TmVx-jo (AUWyri-ix-iy
where M1 is an alkali metal, M2 is an alkaline earth metal, Tm is a transition metal having a valence state, i, x + y = l, and O
Referring to Figure 2, the addition of Se in amounts exceeding 2mol% substantially reduces the hydrogen storage capacity of the metal alanate material. Under hydrogenation conditions of 100°C and 60-68 atm ultra high purity, UHP, hydrogen, NaAlH doped with 3.3mol% Se added in the form of ScClj shows a total hydrogen storage capacity of approximately 1.5wt% after 10 hours. This is shown by the curve 20. Under the same conditions, NaAlH» doped with 2.0mol% Se added in the form of ScCla yields a total hydrogen storage capacity of 4.00-4.50wt% after 10 hours. This is shown by the curve 22. Accordingly, any additional Se catalyst over 2.0mol% has a negative effect on hydrogen storage capacity.
The decreased hydrogen storage capacity in metal alanates with Se levels exceeding 2mol% is more than expected due to the weight of the catalyst itself. For a fixed volume of metal alanate, a catalyst displaces a portion of the hydrogen storing base metal alanate material. Consequently, the use of a catalyst involves competing interests; the beneficia! catalytic effect versus reduced hydrogen capacity from displaced base metal alanate. One skilled in the art can calculate the expected loss in hydrogen storage capacity due to the catalyst displacing the base metal alanate.
When increasing the amount of Se catalyst from 2.0mol% to 3.3mol%, there is an expected loss of hydrogen storage capacity due to the catalyst displacing the base metal alanate. The actual loss of hydrogen storage capacity is greater mân the expected loss. Therefore, the Se must also be acting as a thermodynamic inhibitor to the base metal alanate. This is mainly due to an increase in equilibrium pressure from the "excess" Se dopant.
With this invention, increased performance is possible, and the decreasing effectiveness of increased metal dopants is avoided.
In one example metal alanate material, approximately 0.5mol%-30mol% of dopant oxygen lowers the equilibrium pressure associated with the Se dopant The dopant oxygen lowers the equilibrium pressure and allows a Se dopant to be added at levels exceeding the previoosly effective limits (i.e., 2mol%). în some examples, the Se dopant may be added at levels up to approximately 25mol%. The dopant oxygen counteracts the increase in equilibrium pressure associated with the
increased Se dopant (i.e., an amount over 2mol%) and yields favorable hydrogenation characteristics.
Referring to curve 20 in Figure 2 for example, the hydrogen storage capacity of NaAIHt with a Se dopant added in the form of ScCl3 at 3.3mol% is approximately 1.50%. The storage capacity of NaAlHU with the same amount of Se dopant added in the form of ScCl3 and dopant oxygen added in the form of Na2O, however, is 4.50-5.00%. This is shown by curve 24. The dopant oxygen counteracted the increased equilibrium pressure associated with the Se catalyst. Similar results follow for catalysts other than Se.
Improved results are available even when using the previously believed optimum Se dopant amount The curve 26 shows that adding 0.67mol% Sc2C>3 in addition to 2mol% ScQ3 increases the absorbed hydrogen to more mân 4.5wr%, compared to just over 4.0wt% absorbed hydrogen for 2mol% ScCl3 shown by curve 22. In this example an additional 0.5wt% absorption becomes possible because of the added oxygen dopant.
Severa! different known methods may be used to dope a base metal alanate material with oxygen. In one example high energy ball-milling is one preferred method, using solid oxides or hydroxides as the oxygen source.
The preferred solid oxide oxygen sources include an unstable oxide, such as those having -AG°f 2, CsNO3, CuCNOsk, Fe(N03)2, KNQ3, LiNOa, NaNO3, NH4NO3, NiCNOak, Pb(NO3)2, RbNO3, and Zn (NO3)2. Example suitable carbonates include CdCOs, CoCO3, CuCO3, FeCOs, PbCOs, MnCX)3, NajCO3 and ZnCOs. Another means of incorporating oxygen can be through hydroxides. Example hydroxides include , CsOH, CuCOH) KOH, liOH, MhCOEfc, N2OH, NiCOHfc, PbCOHfc, tCOH, RbOH, Sn(OH)2, TlCOBQs and ZnCOH. When an unstable oxide or hydroxide is ball-milled with a base metal alanate material, the oxide compound disassociates and the oxygen dopes into the metal alanate base material or is otherwise incorporated into the compound. One skilled in the art who has the
benefit of this description will recognize additional suitable unstable solid oxides, mixed oxides or hydroxides.
Another method of doping a base material with oxygen is via an oxygen gas mixture. Oxygen may be introduced into a base metal alanate material through parţial oxidation using oxygen gas in mixture with a non-reactive gas such as Na or Ar.
The invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Vaxious modifications and variations of the given examples are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
1. A reversible hydrogen storage composition comprising:
b. 2.0 to 25 mol% of ScCI3, based on NaAIH4; and
c. 0.5 to 30 mol% of a solid oxide dopant, based on NaAIH4, selected
from an unstable metal oxide having ∆Gf° 2. The reversible hydrogen storage composition as claimed in claim 1, wherein
the unstable metal oxide is selected from the group comprising of BaO2,
BeO, Bi2O3, CdO, Cu2O, Au2O3, IrO2, Li2O, Hg2O, NiO, Tl2O, SeO2, TeO2,
Ag2O, PuO2, PdO, Na2O and ZnO.
3. The reversible hydrogen storage composition as claimed in claims 1-2, is
used in a fuel cell device.
|Indian Patent Application Number||2339/DELNP/2007|
|PG Journal Number||03/2012|
|Date of Filing||26-Mar-2007|
|Name of Patentee||UTC POWER CORPORATION|
|Applicant Address||195 GOVERNOR'S HIGHWAY, SOUTH WINDSOR, CT 06074 USA|
|PCT International Classification Number||C01B 6/24|
|PCT International Application Number||PCT/US2005/033997|
|PCT International Filing date||2005-09-27|