Abstract:
A hydrogen storage material and process is provided in which catalyzed alkali borohydride materials and partially substituted borohydride materials are created and which may contain effective amounts of catalyst(s) which include transition metal oxides, halides, and chlorides of titanium, zirconium, tin, vanadium, iron, cobalt and combinations of the various catalysts and the destabilization agents which include metals, metal hydrides, metal chlorides and complex hydrides of magnesium, calcium, strontium, barium, aluminum, gallium, indium, thallium and combinations of the various destabilization agents. When the catalysts and destabilization agents are added to an alkali borodydride such as a lithium borohydride, the initial hydrogen release point of the resulting mixture is substantially lowered. Additionally, the hydrogen storage material may be rehydrided with weight percent values of hydrogen of at least about nine percent.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Utility application 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. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002]     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. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention is directed towards a hydrogen storage material and process of using the hydrogen storage material in which metal borohydrides may be catalyzed OR destabilized so as to achieve a lower hydrogen release start point of less than 100° C. Further, the present invention is directed to the catalyzed or destabilized borohydrides which may reversibly absorb and desorb hydrogen. A further aspect of the invention is directed to a process of incorporating catalysts or destabilized agents into a metal borohydride so as to achieve novel borohydride compositions having improved hydrogen release kinetics along with an ability to reversibly absorb and desorb hydrogen.  
       BACKGROUND OF THE INVENTION  
       [0004]     This invention relates to the use of borohydrides in hydrogen storage and release technologies. Borohydrides such as LiBH 4  can be used for hydrogen storage and energy systems making use of stored hydrogen. Borohydrides contain a large amount of hydrogen within their molecular structure. For example, LiBH 4  contains 18wt % hydrogen, an amount higher than any other known hydrogen storage material. Accordingly, borohydrides have great potential to be developed as hydrogen storage media.  
         [0005]     Unfortunately, borohydrides release hydrogen at very high temperatures, with temperatures usually exceeding the melting point of the borohydrides. For example, commercially available LiBH 4  releases hydrogen above 400° C. In addition, the hydrogen release mechanism is typically irreversible for commercially available LiBH 4  in that the borohydride cannot be rehydrided.  
         [0006]     It is known to use various borohydrides for specialized applications requiring a hydrogen storage material. For instance, U.S. Pat. No. 6,737,184 assigned to Hydrogenics Corporation, and which is incorporated herein by reference, discloses one release mechanism using LiBH 4  in which a solvent such as water is used to bring about the release of stored hydrogen. However, once released, the LiBH 4  cannot be easily rehydrided.  
         [0007]     Similar aqueous based release reactions for borohydrides may also be seen in reference to U.S. Pat. Nos. 6,670,444; 6,683,025; and 6,706,909 all assigned to Millennium Cell and which are incorporated herein by reference. The cited references are all directed to aqueous-based reactions for releasing hydrogen from a borohydride. There is no discussion within the references of catalysts or material handling techniques that allow the reversible release of hydrogen from a metal borohydride containing solid compound.  
         [0008]     It is also known in the art that borohydrides may release hydrogen through a thermal decomposition process. For instance, in U.S. Pat. No. 4,193,978 assigned to Comphenie Francaise de Raffinage and which is incorporated herein by reference, lithium borohydride is described as a hydrogen storage material which releases hydrogen during a thermal decomposition process. The reference states that aluminum may be added to the lithium borohydride to lower the reconstitution temperature and to increase the hydrogen capacity of the material. There is no discussion of catalysts or other materials or techniques designed to bring about a lower hydrogen release point temperature.  
         [0009]     It has been reported in the article, “Hydrogen Storage Properties of LiBH 4   ”, Journal of Alloys &amp; Compounds,  356-357 (2003) 515-520 by Zuttlel et al and which is incorporated herein by reference, that LiBH 4  may include a low temperature structure of an orthorhombic, space group having a hydrogen desorption value reportedly occurring at approximately 200° C. in the presence of SiO 2 . However, an ability to rehydride the dehydrided lithium borohydride and the use of additives other than the SiO 2  in reducing the dehydriding temperature and isothermal dehydriding properties is not reported.  
         [0010]     Currently, the art recognizes that borohydrides, when subjected to high temperatures, may decompose and release hydrogen at a point in excess of the borohydride&#39;s melting point of 280° C. Alternatively, borohydrides can also be used through an irreversible hydrolysis process to provide a source of hydrogen. However, there remains room for improvement and variation within the art directed to the use of borohydrides in hydrogen storage applications.  
       SUMMARY OF THE INVENTION  
       [0011]     It is one aspect of at least one of the present embodiments to provide for a mixture of a borohydride and an effective amount of a catalyst which reduces the temperature at which stored hydrogen gas is released from the borohydride mixture.  
         [0012]     It is an additional aspect of at least one of the present embodiments of the invention to provide for an effective amount of a catalyst which, when added to a borohydride mixture, enables the resulting mixture to release hydrogen gas and to subsequently be rehydrided under conditions of temperature and pressure.  
         [0013]     It is a further aspect of at least one of the present embodiments of the invention to provide for a hydrogen storage material comprising a mixture of an alkali borohydride with an effective amount of a catalyst selected from the group consisting of TiO 2 , ZrO 2 , SnO 2 , TiCl 3 , SiO 2 , V 2 O 3 , Fe 2 O 3 , MoO 3 , CoO, ZnO, transition metal oxides, halides, and combinations thereof.  
         [0014]     It is a further aspect of at least one of the present embodiments of the invention to provide for a hydrogen storage material comprising a mixture of a borohydride, such as LiBH 4 , with an effective amount of a catalyst selected from the group consisting of TiO 2 , ZrO 2 , SnO 2 , TiCl 3 , SiO 2 , V 2 O 3 , Fe 2 O 3 , MoO 3 , CoO, ZnO, transition metal oxides, halides, and combinations thereof.  
         [0015]     It is a further aspect of at least one embodiment of the present invention to provide for destabilized metallic borohydrides having reduced dehydriding temperatures and improved hydrogen binding/release kinetics by providing a metal borohydride; substituting metal cations (such as Li + , Na + , and K + ) of the metal borohydrides with metal cations having a lower metallic character (such as Mg +2 , Ca +2 , Sr +2 , and Ba +2 ), thereby lowering the stability of BH bonds in a tetrahedron [BH 4 ] −1 ; optionally substituting boron atoms in the tetrahedron with other elements selected from the group consisting of Al, Ga, In, Ti, and combinations thereof; thereby providing a substituted metal borohydride having improved hydrogen kinetics.  
         [0016]     It is a further aspect of at least one embodiment of the present invention to provide for a process of forming a metal borohydride comprising the steps of: providing a supply of LiBH 4 ; mixing with the LiBH 4  a substitution agent selected from the group consisting of metals (such as Mg, Ca, Sr, Ba, and Al by way of non-limiting examples), metal chlorides (such as MgCl 2 , AlCl 3 , CaCl 2 , and TiCl 3 , by way of non-limiting examples), metal hydrides (such as MgH 2 , AlH 3 , CaH 2 , TiH 2 , and ZrH 2  by way of non-limiting examples), complex hydrides (such as LiAlH 4 , NaAlH 4 , and Mg(AlH 4 ) 2  by way of non-limiting examples) and mixtures thereof; ball milling the LiBH 4  and one or more substitution agents; sintering the product of the ball milling at a temperature below the melting point of LiBH 4  and at a hydrogen atmosphere pressure higher than the decomposition pressure of LiBH 4  at said selected temperature, thereby achieving partial substitution by solid diffusion of a lithium component with at least one of said substitution agents, thereby providing a sintered block of partially substituted borohydride; physically reducing the sintered block by crushing and ball milling so as to achieve a nanoscale particle size; and optionally introducing a catalyst to said sintered block during the crushing and ball milling steps.  
         [0017]     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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.  
         [0019]      FIG. 1  is a graph showing the dehydriding characteristics of the indicated catalyzed borohydrides and accompanying control LiBH 4 .  
         [0020]      FIG. 2  is a graph showing the rehydriding capability of the catalyzed borohydrides at 600° C. and 100 bar.  
         [0021]      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.  
         [0022]      FIG. 4  is a graph setting forth desorption data for LiBH 4  75%-TiO 2  25% at respective temperatures of 400° C., 300° C., and 200° C.  
         [0023]      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 .  
         [0024]      FIG. 6  is a graph comparing dehydrogenation of the destabilized and commercial LiBH 4  materials.  
         [0025]      FIG. 7  is a Raman spectra comparison between the destabilized and commercial LiBH 4  materials.  
         [0026]      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.  
         [0027]      FIG. 9  is a graph comparing dehydrogenation of a destabilized LiBH 4  with a commercial LiBH 4  material.  
         [0028]      FIG. 10  is a graph setting forth the desorption data for a partially substituted LiBH 4  material.  
         [0029]      FIG. 11  is a graph showing the rehydriding capability of the partially substituted borohydride material at 600° C. and 70 bars of pressure.  
         [0030]      FIG. 12  is a graph setting forth desorption data for a partially substituted LiBH 4  with the indicated catalyst.  
         [0031]      FIG. 13  is a graph setting forth rehydriding capabilities of the partially substituted borohydride and indicated catalyst.  
         [0032]      FIG. 14  is a graph setting forth desorption data for a partially substituted LiBH 4  with 0.2 molar aluminum.  
         [0033]      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.  
         [0034]      FIG. 16  is a graph setting forth desorption data for LiBH 4  plus 0.5 LiAlH 4 .  
         [0035]      FIG. 17  is a graph setting forth rehydriding capability of a LiBH 4  as partially substituted with 0.5 LiAlH 4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0036]     Reference will now be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. Other objects, features, and aspects of the present invention are disclosed in the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions.  
         [0037]     In describing the various figures herein, the same reference numbers are used throughout to describe the same material, apparatus, or process pathway. To avoid redundancy, detailed descriptions of much of the apparatus once described in relation to a figure is not repeated in the descriptions of subsequent figures, although such apparatus or process is labeled with the same reference numbers.  
         [0038]     In accordance with the present invention, it has been found that borohydrides such as alkali borohydrides may be catalyzed with effective amounts of various oxides and chlorides of titanium, zirconium, tin along with transition metal oxides and other metal and non-metal oxides, halides, and combinations of catalysts so as to reduce the temperature release point for hydrogen. Additionally, the incorporation of effective amounts of catalysts in a mixture with the borohydrides has been found to permit the rehydriding of hydrogen into the mixture material under conditions of elevated temperatures and pressures.  
                                                                                 TABLE 1                                       Weight   Balls   Speed   Milling Time   Temp                Sample   Composition %   (g)   (φmm/#)   (rpm)   (h)   run/rest/cy   (C. °)   Atmosphere               1   75 wt % LiBH 4  + 25 wt % TiO 2     1.00   11/3   600   20   (1 × 0.5 × 20)   25   Ar       2   75 wt % LiBH 4  + 25 wt % ZrO 2     1.00   11/3   600   20   (1 × 0.5 × 20)   25   Ar       3   75 wt % LiBH 4  + 25 wt % SiO 2     2.00   11/3   600   20   (1 × 0.5 × 20)   25   Ar       4   75 wt % LiBH 4  + 25 wt % SnO 2     2.00   11/3   600   20   (1 × 0.5 × 20)   25   Ar       5   75 wt % LiBH 4  + 25 wt % TiCl 3     2.00   11/3   600   20   (1 × 0.5 × 20)   25   Ar                  
 
         [0039]     As seen in reference to Table 1, the indicated weight percent of lithium borohydride was mixed with a 25wt % of the indicated oxide or chloride of Ti, Si, Zr, and/or Sn. The indicated amounts of the resulting compositions were subjected to a ball milling process using three 11 mm diameter tungsten carbide balls in conjunction with a Fritsch ball mill apparatus. Samples of lithium borohydrides dried in an inert argon atmosphere were transferred inside the argon glovebox to two 45 ml grinding jars of the Fritsch ball mill apparatus, which were then sealed for protection during transfer to the Fritsch ball mill apparatus. At all times during the ball milling process, the borohydride and respective catalysts were maintained in an inert argon atmosphere. The ball mill apparatus was operated at 600 rpms. The ball milling times, as indicated, extended up to 20 hours using a cycle of 1 hour run time followed by a half hour of rest. The ball milling apparatus was run at ambient temperatures of 25° C.  
         [0040]     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).  
         [0041]     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.  
         [0042]     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.  
         [0043]     As indicated by the data set forth below, the catalyzed borohydride compounds exhibit a hydrogen release initiation temperature which is reduced from 400° C. to 200° C. Additionally, the catalyzed borohydrides have shown a reversible capacity of about 6wt % to about 9wt % hydrogen. However, as the catalyst amounts and ball milling processes are optimized, it is envisioned that cycles of rehydrating and dehydrating will result in the reversible release of even greater weight percent amounts of hydrogen. The ability to rehydride borohydrides at the demonstrated temperatures and pressures represents a significant improvement and advancement within the art. The reversible capacity for hydrogen storage, when combined with the demonstrated ability of reduced temperature release kinetics, are significant advancements within the area of hydrogen storage materials in particular for borohydrides.  
         [0044]     As seen in reference to  FIG. 4 , the sample 1 of LiBH 4  75%-TiO 2  25% desorbs 8.5wt %, 5.0wt %, and 1.5wt % hydrogen at 400° C., 300° C., and 200° C. respectively. It is expected that the lower dehydriding temperature and the higher dehydriding capacity are achievable through the optimization of the catalysts, catalyst loading and synthesis parameters.  
         [0045]     As seen in reference to  FIG. 5 , sample 1 of LiBH 4  75%-TiO 2  25% has a unique crystal structure that differs from the original LiBH 4 .  
         [0046]     As seen in reference to  FIG. 1 , five specific catalysts (samples 1-5) have been seen to be effective in reducing dehydrating temperatures and producing a reversible hydrogen storage material. It is recognized and understood that the operative amounts of catalysts and the conditions for combining the catalysts with the borohydrides have not been optimized. While 25wt % loadings of various catalysts have proven effective, as various catalysts are evaluated and optimized, it is believed that catalyst amounts as low as about 10wt % to as high as about 50wt % may offer optimal results. It is well within the skill level of one having ordinary skill in the art to use routine experimentation to determine the preferred and optimal amounts of catalysts using the techniques described herein and thereby determine the most effective weight percent amounts of catalyst.  
         [0047]     Similarly, the equipment and resulting processes used to carry out the ball milling process as well as the Temperature Programmed Desorption (TPD) parameters can also be refined. Again, it is believed that variations in the ball milling process, such as the parameters of ball number, size, weight, and ball milling speed may be varied to achieve the desired results.  
         [0048]     According to another aspect of at least one embodiment of the present invention, it has been found that the borohydrides, such as LiBH 4 , NaBH 4 , and KBH 4  may be modified through partial substitution with one or more destabilization agents to result in a lower dehydriding temperature and improved dehydriding and rehydriding kinetics. As used herein, the term “destabilization agent” includes an element or molecule which is partially substituted for either the lithium atom or the boron atom within a borohydride such as LiBH 4 . A non-limiting example of some suitable substitution agents includes metals such as magnesium, aluminum; metal chlorides such as MgCl 2 , CaCl 2 , AlCl 3 , TiCl 3 , and FeCl 3 ; metal hydrides such as MgH 2 , CaH 2 , AlH 3 , TiH 2 , and ZrH 3 ; and complex hydrides such as LiAlH 4 , NaAlH 4 , and Mg(AlH 4 ) 2 ; and combinations thereof. While not wishing to be limited by theory, Applicant believes that the substitution agents, as seen by the non-limiting examples provided above, have less ionic character than the original metal borohydrides. As a result, the partial substitution of metal cations by cations having a lower ionic property reduces the ionic strength of the bond between the metal B and the hydrogen. The hydrogen atoms are thus more easily removed, indicative of the lower stability of the B—H bonds in the tetrahedrons [BH 4 ] −1 . It is further believed that the binding strength of the B—H bonds within the tetrahedron can be reduced when the boron atom is partially substituted by another element such as Al, Ga, In, Ti, Zr, or V.  
         [0049]     As set forth below, it has been demonstrated that various metals, metal chlorides, metal hydrides, and other complex hydrides may be used as destabilization agents 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. A Mechano-Thermal Diffusion Process (MTDP) of achieving the partial substitution is as follows:  
         [0050]     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 MgH2, 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.  
         [0051]     Step 2. Following the initial ball milling and mixing, the resulting mixture is sintered at a temperature (300° C.) below the melting point of LiBH 4  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. It is believed that partial substitution takes place through solid diffusion of the elements and the subsequent lattice reconfiguration. It has been found that the sintering conditions described above for a length of time of 5 to 10 hours is sufficient to achieve partial substitution means that a percentage less than 100% of the Li and/or B are substituted by the additives introduced above.  
         [0052]     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.  
       EXAMPLE 1  
       [0053]     Using the protocol set forth above, LiBH 4  was mixed with 0.2 molar magnesium and used to obtain the partial substitution. As seen in reference to  FIGS. 6 through 8 , the destabilized material LiBH 4 +0.2Mg releases hydrogen at 60° C. comparing with the commercial pure LiBH 4  that releases hydrogen at 325° C. At room temperature, two Raman active internal BH 4   −1  vibrations v 4  and v′ 4  occur at 1253 and 1287 cm −1  respectively, and two overtones 2v 4  and 2v 4 ′ at 2240 and 2274 cm −1 , respectively as spectrum 2 shows in  FIG. 7 . However, the V 4  v′ 4 , and 2v 4  stretching disappears from the spectrum after the addition of the destabilized LiBH 4 +0.2 Mg. The 2v 4 ′ stretching is weakened and shifted to 2300 cm 1  as the spectrum 1 shows and is indicative that the B—H binding strength is reduced by partial LI +1  substitution. The weakened bond results in a lower dehydriding temperature. As further seen in reference to  FIG. 8 , the partially substituted LiBH 4  material is able to undergo multiple cycles of rehydrogenation.  
       EXAMPLE 2  
       [0054]     LiBH 4  was combined with 0.3 MgCl 2  plus 0.2 molar TiCl 3  and is subjected to the MTDP substitution process described above. As seen from data set forth in  FIG. 9 , the partially substituted product has improved hydrogen desorption release properties in terms of temperature and percent of hydrogen released at temperatures below 500° C. when compared to a commercial LiBH 4 .  
         [0055]     As set forth in  FIGS. 10 and 11 , data is set forth showing the repeated desorption and rehydrogenation capabilities respectively of the partially substituted LiBH 4 .  
       EXAMPLE 3  
       [0056]     LiBH 4  was mixed with 0.5 MgH 2  plus 0.007 TiCl 3  and processed according to the MTDP substitution steps described above. Set forth in  FIG. 12  is the hydrogen desorption data of the resulting product at the indicated temperatures.  
         [0057]     In  FIG. 13 , rehydrogenation data of the partially substituted LiBH 4  is set forth.  
       EXAMPLE 4  
       [0058]     LiBH 4  at 80wt % was combined with 0.2 molar Al and treated with the MTDP substitution protocol described above. As set forth in  FIGS. 14 and 15 , the data on hydrogen desorption and rehydrogenation respectively is provided.  
       EXAMPLE 5  
       [0059]     LiBH 4  was combined with 0.5 LiAlH 4  and subjected to the MTDP substitution protocol described above. As seen in reference to  FIGS. 16 and 17 , the respective hydrogen desorption and rehydrogenation properties of the partially substituted LiBH 4  are provided.  
         [0060]     As seen from the above examples, it is possible to use destabilization agents to partially substitute a percentage of either Li atoms or B atoms in LiBH 4  (or both atoms) and thereby achieve a lower dehydriding temperature than is otherwise possible using non-substituted LiBH 4 . In addition, as noted by the data set forth in the Figures, favorable dehydriding and rehydriding kinetics can be obtained using the partial substitution protocol along with the optional addition of catalysts such as TiCl 3  or TiO 2 .  
         [0061]     Although preferred embodiments of the invention have been described using specific terms, devices, and methods, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention which is set forth in the following claims. In addition, it should be understood that aspects of the various embodiments may be interchanged, both in whole, or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.