Destabilized and catalyzed borohydride for reversible hydrogen storage

This application is a continuation-in-part of U.S. Ser. No. 12/035,710 filed on Feb. 22, 2008 and a continuation-in-part of U.S. Ser. No. 11/130,750 filed on May 17, 2005, and which claims the benefit of U.S. Provisional Application No. 60/605,177, filed on Aug. 27, 2004, the specifications of which are incorporated herein by reference.

This invention was made with Government support under Contract No. DE-AC0996-SR18500 awarded by the United States Department of Energy. The Government has certain rights in the invention.

The invention relates to hydrogen storage materials, and with more particularity, to hydrogen storage materials having improved thermodynamic properties.

Current technologies utilized for gaseous hydrogen storage are limited by the low-volume storage gas density even at very high pressures, such as pressures in the range of 5,000 to 10,000 psi. The energy density by volume of the gaseous hydrogen is less than that of a gasoline energy density. Use of hydrogen as an alternate fuel is limited due to this lower energy density. Cryogenic storage of hydrogen at temperatures of around 20 K may improve the volumetric energy density compared to gaseous storage, but is still less than that for a given amount of energy when compared to gasoline. Additionally, production of liquid hydrogen is energy intensive and requires special considerations due to the low temperature storage to avoid hydrogen boil off and other limitations of liquefied hydrogen.

Chemical storage of hydrogen in a solid, such as borohydride, allows for hydrogen release when heated or mixed with water. However, formation of solid byproducts or release of hydrogen at very high temperatures, usually exceeding the melting point of the borohydride, limit the use of borohydrides. Additionally, borohydrides are not typically able to be rehydrided after hydrogen release.

There is therefore a need in the art for an improved hydrogen storage material that releases hydrogen at lower temperatures and is able to be rehydrided after release of the hydrogen.

In one aspect, there is disclosed a process of forming a hydrogen storage material, including the steps of: providing a first material of the formula M(BH 4 ) X , where M is an alkali metal or an alkali earth metal, providing a second material selected from M(AlH 4 ) x , a mixture of M(AlH 4 ) x and MCl x , a mixture of MCl x and Al, a mixture of MCl x and AlH 3 , a mixture of MH x and Al or AlH 3 , Al, and AlH 3 . The first and second materials are combined at an elevated temperature and at an elevated hydrogen pressure for a time period forming a third material having a lower hydrogen release temperature than the first material and a higher hydrogen gravimetric density than the second material.

A fully enabling disclosure of the present invention, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings.

FIG. 1 is a graph showing the dehydriding characteristics of the indicated catalyzed borohydrides and accompanying control LiBH 4 .

FIG. 2 is a graph showing the rehydriding capability of the catalyzed borohydrides at 600° C. and 100 bar.

FIG. 3 is a graph setting forth the first and second cycle hydrogen release characteristics of LiBH 4 75%-TiO 2 25% at the indicated temperatures.

FIG. 4 is a graph setting fort desorption data for LiBH 4 75%-TiO 2 25% at respective temperatures of 400° C., 300° C., and 200° C.

FIG. 5 is an x-ray diffraction spectra setting forth the unique crystal structure of LiBH 4 75%-TiO 2 25% in comparison to a sample of LiBH 4 .

FIG. 6 is a graph comparing dehydrogenation of the destabilized and commercial LiBH 4 materials.

FIG. 7 is a Raman spectra comparison between the destabilized and commercial LiBH 4 materials.

FIG. 8 is a graph setting forth the first, second, and third cycle hydrogen release characteristics of a partially substituted LiBH 4 in which the substituted material is LiBH 4 plus 0.2 molar Mg.

FIG. 9 is a graph comparing dehydrogenation of a destabilized LiBH 4 with a commercial LiBH 4 material.

FIG. 10 is a graph setting forth the desorption data for a partially substituted LiBH 4 material.

FIG. 11 is a graph showing the rehydriding capability of the partially substituted borohydride material at 600° C. and 70 bars of pressure.

FIG. 12 is a graph setting forth desorption data for a partially substituted LiBH 4 with the indicated catalyst.

FIG. 13 is a graph setting forth rehydriding capabilities of the partially substituted borohydride and indicated catalyst.

FIG. 14 is a graph setting forth desorption data for a partially substituted LiBH 4 with 0.2 molar aluminum.

FIG. 15 is a graph showing the rehydriding capability of the partially substituted LiBH 4 referred to in FIG. 14 at 600° C. and 100 bars of hydrogen pressure.

FIG. 16 is a graph setting forth desorption data for LiBH 4 plus 0.5 LiAlH 4 .

FIG. 17 is a graph setting forth rehydriding capability of a LiBH 4 as partially substituted with 0.5 LiAlH 4 .

In one aspect of the invention, a hydrogen storage material may be formed from a first material, such as a metal containing borohydride, where the metal may be an alkali metal or an alkali earth metal. The first material may have the formula: M(BH 4 ) x where M is an alkali metal or an alkaline earth metal and 1≦x≦2.

The first material may be combined with a second material, such as a metal alanate of the formula: M(AlH 4 ) x where 1≦x≦2, a mixture of the metal alanate and a metal chloride, a mixture of a metal chloride and aluminum, a mixture of a metal chloride and an alane (AlH 3 ), a mixture of a metal hydride of the formula: MH x where 1≦x≦2 and aluminum or alane, aluminum, and an alane.

The first and second materials may be combined at an elevated temperature and at an elevated hydrogen pressure for a time period to form a material having a lower hydrogen release temperature than the first material and a higher hydrogen gravimetric density than the second material.

Various metal borohydrides may be utilized as the first material, including lithium borohydride, sodium borohydride, potassium borohydride, or combinations of the above materials. Additionally, various alkali earth metals may be included in the metal borohydride and may be selected from magnesium, calcium, strontium, barium, aluminum, and mixtures of the above.

Various metal chlorides may be used in the second material as described above. Such metal chlorides may include including magnesium chloride, calcium chloride, strontium chloride, barium chloride, zirconium chloride, titanium chloride and combinations thereof. It should be realized that while chlorides are outlined for use in certain embodiments various metal halides including bromides and iodides may also be used.

Various metal hydrides may be utilized in the second material as described above. Such metal hydrides may include magnesium hydride, calcium hydride, titanium hydride, and zirconium hydride, and combinations thereof.

As stated above, various alanates may be used where the metal is selected from an alkali metal or an alkali earth metal and may include lithium alanate having the formula LiAlH 4 , sodium alanate having the formula NaAlH 4 , and magnesium alanate having the formula Mg(AlH 4 ) 2 .

In one aspect, the process of forming the hydrogen storage material may include the step of ball milling the first and second materials prior to the step of combining the first and second materials. In the ball milling process, the first and second materials may be introduced into a ball mill and milled to a particle size ranging from about 50 to 100 nanometers.

In one embodiment, a first material or metal borohydride, such as lithium borohydride and a second material, an alanate, may be combined using the ball mixing or milling procedure. Following the ball milling procedure, the mixed material may be subjected to a high temperature treatment at a temperature of up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of time up to 24 hours. The third material formed from the process may have a lower hydrogen desorption temperature and faster desorption kinetics compared to the initial metal borohydride materials. Additionally, the third material of the process may be reversibly hydrogenated after release of an initial hydrogen composition. The third material formed by the process may contain a partially substituted borohydride lithium metal cation with the alanate metal cation, or partially substituted borohydride boron with aluminum, or a partially substituted cation and boron in the borohydride.

In another embodiment, the first material may be a metal borohydride, such as lithium borohydride and the second material may include an alanate and a metal halide such as titanium chloride that may be mixed using the ball mixing procedure. Following the mixing, the first and second materials may be combined at an elevated high temperature of up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of time up to 24 hours. The third material formed by the high temperature treatment may have a lower hydrogen desorption temperature and faster desorption kinetics compared to the initial borohydride materials. The third material formed may be reversibly hydrogenated when an initial hydrogen is removed from the composition. The third material may contain a partially substituted borohydride lithium metal cation with the alanate metal cation, or partially substituted borohydride boron with aluminum, or a partially substituted cation and boron in the borohydride.

In another embodiment, the first material may be a metal borohydride, such as lithium borohydride, and the second material may be a metal halide, such as zirconium chloride or titanium chloride, magnesium chloride or calcium chloride, and aluminum or an alane may be mixed using the ball mixing procedure. The first material and second material may be combined at an elevated temperature of up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of time of up to 24 hours. The third material formed by the high temperature treatment may have a lower hydrogen desorption temperature and faster desorption kinetics compared to the initial borohydride materials. Additionally, the third material of the process may be reversibly hydrogenated after release of an initial hydrogen composition. The third material may include a partially substituted borohydride lithium metal cation with the halide cation, and/or a partially substituted borohydride boron with aluminum, or a partially substituted cation and boron in the borohydride.

In another embodiment, the first material may be a metal borohydride, such as lithium borohydride, and the second material may be a hydride, such as an alkali earth based hydride, such as magnesium hydride, calcium hydride, or a transition metal hydride, such as zirconium hydride, titanium hydride, and aluminum or alane (AlH 3 ), may be mixed using the ball mixing and milling procedure. The first and second materials may be combined in a high temperature treatment at temperatures of up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of time of up to 24 hours. As with the previously described embodiments, the third material of the process may have lower hydrogen desorption temperatures and faster desorption kinetics compared to the first material, and may be reversibly hydrogenated. The third material may be a partially substituted borohydride cation and/or a partially substituted borohydride boron with aluminum, or both a partially substituted cation and anion in the borohydride.

In another embodiment, the first material may be a metal borohydride, such as lithium borohydride, and the second material may be an alane and may be mixed using the ball mixing procedure. The first and second materials may be combined in a high temperature treatment at a temperature of up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of up to 24 hours. The third material of the process may have lower hydrogen desorption temperatures and faster desorption kinetics compared to the first material. Additionally, the third material may be reversibly hydrogenated. The third material may include a partially substituted borohydride cation with aluminum, and/or a partially substituted borohydride boron with aluminum.

In another embodiment, the first material may be a metal borohydride, such as lithium borohydride, and the second material may be aluminum and may be combined using the ball mixing procedure. The ball mixing procedure may be followed by a high temperature treatment at temperatures up to 300° C. under hydrogen pressures of up to 5,500 psi for a period of time of up to 24 hours. The third material of the process may have lower hydrogen desorption temperatures and faster desorption kinetics compared to the first material. The third material may also be reversibly hydrogenated. The third material may include a partially substituted borohydride cation with aluminum, and/or a partially substituted borohydride boron with aluminum.

Following the ball milling process, mixture samples ranging from approximately 0.250 grams to approximately 0.500 grams were evaluated in a Sieverts volumetric apparatus using a Temperature Programmed Desorption (TPD) from ambient temperature to 600° C. with a heating rate of 5° C./min. The desorption conditions included a backpressure of P 0 =5.4 mbar. The results of the hydrogen desorption are set forth in FIG. 1 as samples 1-5 corresponding to Table 1 along with the appropriate control of commercially available LiBH 4 (100%) (Sample 6).

Following the hydrogen desorption, the desorbing material was rehydrided at 600° C. and 100 bar of hydrogen for 45 minutes. As indicated in FIG. 2 , the percent of hydrogen absorbed for the indicated materials is reflected on the Y axis.

As seen in FIG. 3 , the sample of LiBH 4 75%-TiO 2 25% exhibits reversible hydrogen cycling characteristics as indicated by the capacity in weight percent of the material in a first dehydriding and a second dehydriding cycle.

As indicated by the data set forth in the examples, the third materials exhibit a hydrogen release initiation temperature which is reduced from 400° C. to 200° C. Additionally, the third materials have shown a reversible capacity of about 6 wt % to about 9 wt % hydrogen.

As set forth below, it has been demonstrated that various metals, metal chlorides, metal hydrides, and other complex hydrides may be used as the second materials in the process to substitute a percentage of either the Li atoms or B atoms in LiBH 4 resulting in lower dehydrating temperatures. It is also demonstrated that the partial destabilization may bring about improvements in dehydriding and rehydriding kinetics. The process may include various steps and may include:

Step 1. A mixture of commercial LiBH 4 is combined with metals such as Mg Ca, Sr, Ba, and Al; metal chlorides such as MgCl 2 , CaCl 2 , SrCl 2 , BaCl 3 ; metal hydrides such as MgH 2 , CaH 2 , AlH 3 ; or other complex hydrides such as LiAlH 4 , NaAlH 4 , and Mg(AlH 4 ) 2 ; which are collectively ball milled to achieve a reduced particle size and bring about a homogeneous mixing of the materials.

Step 2. Following the initial ball milling and mixing, the resulting mixture is sintered at a temperature up to 300° C. at a given hydrogen atmosphere (100 bar) such that the hydrogen pressure is greater than the decomposition pressure of LiBH 4 at the reaction temperature.

Step 3. The resulting sintered block of partially substituted material is crushed and ball milled so as to achieve a final average particle size of between about 20 to about 100 nanometers or less. As demonstrated by the data discussed below, during the final ball milling step, catalysts such as TiCl 3 and TiO 2 may be added and which provide for additional improvements in the kinetics and properties of hydrogen absorption and release.