Mixed metal borohydrides

Abstract

The invention relates to mixed metal borohydrides used for solid hydrogen storage. The mixed metal borohydrides are synthesized through solution synthesis using multiple metal borohydrides. First and second precursor solutions are prepared and combined to create a mixture in which the mixed metal borohydride is formed. The solvent is removed, leaving the mixed metal borohydride. The first precursor solution consisting essentially of lithium borohydride, and the second precursor solution consisting essentially of a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium.

Claims

1. A method of producing a mixed metal borohydride comprising: preparing a first precursor solution consisting essentially of a homometallic borohydride compound and a first solvent, the homometallic borohydride being lithium borohydride; preparing a second precursor solution consisting essentially of a second solvent and a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium; combining the first and second precursor solutions to create a mixture; forming the mixed metal borohydride in the mixture by stirring the mixture at room temperature; and removing the first solvent and the second solvent from the mixture containing the mixed metal borohydride to produce a solid mixed metal borohydride; and, wherein the second precursor solution consists essentially of the second solvent and sodium borohydride and titanium borohydride and, wherein the solid mixed metal borohydride is Li.sub.3Ti(BH.sub.4).sub.6.

2. The method of claim 1, wherein the first solvent and the second solvent are the same.

3. The method of claim 1, wherein the first solvent, the second solvent, or both is diethyl ether.

4. The method of claim 1, wherein a third precursor solution is prepared consisting essentially of a third solvent and a third borohydride compound, the third borohydride compound being different from the borohydride compound in the second precursor solution and containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium, and wherein the third precursor solution is combined with the first and second precursor solutions to create the mixture.

5. The method of claim 1, wherein the second precursor solution consists essentially of the second solvent and one or more of sodium borohydride, magnesium borohydride, calcium borohydride and titanium borohydride.

6. The method of claim 1, wherein the second precursor solution consists essentially of the second solvent and calcium borohydride.

7. The method of claim 1, wherein the solvent is removed from the mixture in vacuo.

8. The method of claim 1, wherein the steps of preparing the precursor solutions are done under argon at room temperature.

9. The method of claim 1, wherein a majority of the solid mixed metal borohydride comprises nanoparticles.

10. The method of claim 1, wherein the solid mixed metal borohydride is coated.

11. The method of claim 10, wherein the solid mixed metal borohydride is coated with an inert material.

12. A method of producing a mixed metal borohydride comprising: preparing a first precursor solution consisting essentially of a homometallic borohydride compound and a first solvent, the homometallic borohydride being lithium borohydride; preparing a second precursor solution consisting essentially of a second solvent and a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium; combining the first and second precursor solutions to create a mixture; forming the mixed metal borohydride in the mixture by stirring the mixture at room temperature; and removing the first solvent and the second solvent from the mixture containing the mixed metal borohydride to produce a solid mixed metal borohydride; and wherein the second precursor solution consists essentially of the second solvent and sodium borohydride and titanium borohydride.

13. The method of claim 12, wherein the solid mixed metal borohydride contains less than 5 atomic weight percent of titanium.

14. A method of producing a mixed metal borohydride comprising: preparing a first precursor solution consisting essentially of a homometallic borohydride compound and a first solvent, the homometallic borohydride being lithium borohydride; preparing a second precursor solution consisting essentially of a second solvent and a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium; combining the first and second precursor solutions to create a mixture; forming the mixed metal borohydride in the mixture by stirring the mixture at room temperature; and removing the first solvent and the second solvent from the mixture containing the mixed metal borohydride to produce a solid mixed metal borohydride; and, wherein the second precursor solution consists essentially of the second solvent and sodium borohydride, calcium borohydride and titanium borohydride.

15. The method of claim 14, wherein the solid mixed metal borohydride contains less than 5 atomic weight percent of titanium.

16. A method of producing a mixed metal borohydride comprising: preparing a first precursor solution consisting essentially of a homometallic borohydride compound and a first solvent, the homometallic borohydride being lithium borohydride; preparing a second precursor solution consisting essentially of a second solvent and a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium; combining the first and second precursor solutions to create a mixture; forming the mixed metal borohydride in the mixture by stirring the mixture at room temperature; and removing the first solvent and the second solvent from the mixture containing the mixed metal borohydride to produce a solid mixed metal borohydride; wherein the second precursor solution consists essentially of the second solvent and magnesium borohydride; and, wherein the solid mixed metal borohydride is Li.sub.2Mg(BH.sub.4).sub.4.

17. A method of producing a mixed metal borohydride comprising: preparing a first precursor solution consisting essentially of a homometallic borohydride compound and a first solvent, the homometallic borohydride being lithium borohydride; preparing a second precursor solution consisting essentially of a second solvent and a borohydride compound containing one or more metal cations selected from the group of metals consisting of sodium, magnesium, calcium and titanium; combining the first and second precursor solutions to create a mixture; forming the mixed metal borohydride in the mixture by stirring the mixture at room temperature; and removing the first solvent and the second solvent from the mixture containing the mixed metal borohydride to produce a solid mixed metal borohydride; wherein the second precursor solution consists essentially of the second solvent and magnesium borohydride; and, wherein the solid mixed metal borohydride is Li.sub.3Mg(BH.sub.4).sub.5.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows an illustration of the particle size optimization and encapsulation of the mixed metal borohydrides.

(2) FIGS. 2A-2B show the gravimetric hydrogen density of the homometallic borohydride compounds.

(3) FIGS. 3A-3B describe the precursor compounds and the mixed metal borohydride compounds in FIGS. 2A-2B by the generic formulas.

(4) FIG. 4 shows by combining a compound from Group A with one from Group B in the appropriate stoichiometry and removal of solvent, a wide variety of mixed metal borohydride complexes or reactive borohydride complexes can be synthesized.

(5) FIG. 5 illustrates examples of mixed metal borohydrides that can be created.

(6) FIG. 6 illustrates compounds produced based mixed metal borohydrides.

(7) All descriptions and callouts in the Figures and all content therein are hereby incorporated by this reference as if fully set forth herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The embodiments of the present inventions described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present inventions.

(9) All publications and patents mentioned herein are incorporated herein by reference in their respective entireties for the purpose of describing and disclosing, for example, the constructs and methodologies that are described in the publications which might be used in connection with the presently described invention. The publications discussed above and throughout the text are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventor is not entitled to antedate such disclosure by virtue of prior invention.

(10) It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific processes described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific measurements and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

(11) Unless otherwise specified, the following definitions and methods are used herein: mixed metal borohydrides means a borohydride compound that contains two or more different metals; homometallic borohydride means a borohydride compound containing one metal; nanoparticle, as generally used herein refers to particle or a structure in the nanometer (nm) range, typically from about 0.1 nm to about 1000 nm in diameter; wt % means weight percent; THF means tetrahydrofuran; TGA means thermogravimetric analysis; and GC/MS means gas chromatography/mass spectrometry.

(12) Unless otherwise specified herein, all disclosed characteristics, values and ranges are as determined at room temperature (20 C. to 25 C.).

(13) The alkali metal borohydride compounds LiBH4, NaBH4, and KBH4 are commercially available. The borohydrides of the alkaline earth and light transition metals, Mg(BH.sub.4).sub.2, Al(BH.sub.4).sub.3, Mn(BH.sub.4).sub.2, Zn(BH.sub.4).sub.2, Zr(BH.sub.4).sub.4, Ti(BH.sub.4).sub.3, and Sc(BH.sub.4).sub.3 are for the most part known compounds and their solution synthesis has been described. Briefly, these compounds are prepared as follows according to literature reports. Magnesium Borohydride (Zanella, P.; Crociani, L.; Masiocchi, N.; Biunchi, G. Inorg. Chem. 2007, 46, 9039-9041.); Two methods have been demonstrated for the preparation of magnesium borohydride (Mg(BH.sub.4).sub.2), reaction of commercially available Mg(nBu).sub.2 with borane-methyl sulfide in toluene, and isolation of the precipitate, and reaction of a toluene solution of Al(BH.sub.4).sub.3 (described below) with Mg(nBu).sub.2, and isolation and purification of the precipitate. Both methods furnish the desired product in high yield. Aluminum Borohydride (Zanella, P.; Crociani, L.; Masiocchi, N.; Biunchi, G. Inorg. Chem. 2007, 46, 9039-9041.): A toluene solution of aluminum borohydride (Al(BH.sub.4).sub.3) was prepared in 78% yield by reaction of purified AlC13 with lithium borohydride, followed by distillation of the filtrate. Manganese Borohydride (Makhaev, V. D.; Borisov, A. P.; Gnilomedova, T. P.; Lobkovskii, e. B.; Chekhlov, A. N. Russ. Chem. Bull. 1987, 36(8), 1582-1586.): The THF adduct of manganese borohydride (Mn(BH.sub.4).sub.2.3THF) was obtained by reaction of MnC1.sub.2 with an excess of sodium borohydride followed by filtration and purification to remove excess sodium borohydride. Titanium Borohydride (Mirviss, S. B.; Dougherty, H. W.; Looney, R. W. U.S. Pat. No. 3,310,547 Mar. 21, 1967.): A THF solution of titanium borohydride (Ti(BH.sub.4).sub.3) was prepared by reaction of titanium tetraisopropoxide with diborane in THF. The titanium was reduced from the +4 to +3 valence state during the reaction. When complexed with two and three molecules of THF the titanium compounds are blue and reddish-violet crystalline solids, respectively. Scandium Borohydride (Marks, T. J.; Kolb, J. R. Chem. Rev. 1977, 77, 263-293. Morris, J. H.; smith, W. E. J. Chem. Soc., Chem. Commun. 1970, 245.): Treatment of anhydrous scandium (III) chloride with a slight excess of lithium borohydride in THF furnished a volatile white solid of Sc(BH.sub.4).sub.3.THF. Zinc Borohydride (Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents 1988, Academic Press, ISBN 0-12-549875-6, p 414.): A THF solution of zinc tetrahydroborate (Zn(BH.sub.4).sub.2) was prepared by reaction of two equivalents of sodium borohydride with ZnC1.sub.2 in THF in quantitative yield. The supernatant liquid was free of chloride according to a silver nitrate test. The solution could be stored under N.sub.2 and used as required. Zirconium Borohydride (Marks, T. J.; Shimp, L. A. J. Am. Chem. Soc. 1972, 94, 1542-1550. Williams, D. R.; Benbow, J. W.; Sattleberg, T. R.; Ihle, D. C. Tetrahedron Lett. 2001, 42, 8597-8601.): This compound was prepared neat or as an ethereal solution by reaction of zirconium (IV) chloride with either lithium or sodium borohydride.

(14) As described above, once the mixed metal borohydrides are created, the particle size can be optimized and coating protocols can be used to create a stabilizing shell of inert material, as illustrated in the FIG. 1

(15) 1. FIGS. 2A-2B shows the gravimetric hydrogen density of the homometallic borohydride compounds on the left and some mixed metal borohydride compounds on the right. By combining a compound from one group with another in the appropriate stoichiometry at an appropriate temperature, a wide variety of mixed metal borohydride complexes can be synthesized.

(16) The equations shown below are examples of the reactions to create mixed metal borohydrides.
LiBH.sub.4+Mg(BH.sub.4).sub.2.fwdarw.LiMg(BH.sub.4).sub.3
4LiBH.sub.4+3Mg(BH.sub.4).sub.2.fwdarw.Li.sub.4Mg.sub.3(BH.sub.4).sub.10
Mg(BH.sub.4).sub.2+Ti(BH.sub.4).sub.3.fwdarw.MgTi(BH.sub.4).sub.5

(17) The precursor compounds and the mixed metal borohydride compounds in FIGS. 2A-2B can be described by the generic formulas, such as shown in FIGS. 3A and 3B. The binary (i.e., two metals) complex hydride compositions are derived by the combination of parent compounds from the different groups. However, it should be noted that, as shown in the examples, the mixed metal borohydride compounds can have more than two metals.

(18) For example, FIG. 3A shows groups of homometallic borohydride precursor compounds. Groups A and B can be prepared using solution methods or purchased if commercially available. By combining a compound from Group A with one from Group B in the appropriate stoichiometry and removal of solvent, a wide variety of mixed metal borohydride ate complexes or reactive borohydride complexes can be synthesized, as seen in the list in the block on the right of FIG. 4, where the Na and K cations in parentheses are alternatives to Li. Although illustrated using a 1:1 mole ratio of a compound from Group A with Group B, other mole ratios can be used. The materials can be converted to nanoparticles, or directly prepared as a monodispersion of nanoparticles in the target size range of 5-50 nm. It is believed that particle size can effect the dehydrogenation temperature.

(19) Once nanoparticles of the optimal size are prepared, coating protocols can be used to create a stabilizing shell of inert material. The goal of the stabilization is both chemical, to reduce the flammability of the materials and prevent degradation, and mechanical, to reinforce the material so that any structural changes during dehydrogenation (and potentially, rehydrogenation cycles) are accommodated without loss of reactivity. By keeping the shell layer very thin compared to the size of the core, the impact on gravimetric and volumetric hydrogen density is minimized.

(20) Examples of mixed metal borohydrides that can be created are summarized in FIG. 5. As noted above, having a hydrogen storage compound that can release hydrogen with a 100 to 400 C. range is preferred. Even more preferential is a range of 100 to 250 C. The following compounds are ones that should release hydrogen within the preferred range. As the gravimetric capacity increases, a higher desorption temperature is allowable given the benefit of hydrogen storage and release capacity.

(21) Single experiments were performed for the preparation of Li.sub.2Mg(BH.sub.4).sub.4 and Li.sub.3Mg(BH.sub.4).sub.5 and some characterization was carried out. The Mg(BH.sub.4).sub.2 was prepared using literature methods and blended with a commercial solution of LiBH.sub.4 in ether at low temperature. Upon thermolysis Li.sub.2Mg(BH.sub.4).sub.2 began to evolve H.sub.2 at 130 C. and delivered 10.5 wt % H.sub.2 after heating to 375 C. The Li.sub.3Mg(BH.sub.4).sub.5 sample began to lose H.sub.2 at 166 C. and delivered 7.8 wt % H.sub.2 after heating to about 370 C. The additional weight loss in the TGA is probably residual solvent, as the GC/MS analysis of the headspace only showed trace amounts of solvents and did not show the presence of diborane/pentaborane. Similarly, an attempt was made to prepare Li.sub.3Ti(BH.sub.4).sub.6 by blending a commercial solution of LiBH.sub.4 in ether with Ti(BH.sub.4).sub.3.THF prepared using literature methods (Franz, K.; Fusstetter, H.; Nth, H. Z. anorg. Allg. Chemie 1976, 427(2), 97-113.). Upon thermolysis the tarry aubergine colored material began to release H.sub.2 at 159 C. and delivered 7.3 wt % H.sub.2 after heating to 292 C.

(22) For example, thin polyelectrolyte or polymer-inorganic hybrid coatings can be added using layer-by-layer self-assembly techniques, such as described by Dobbins et al. (Material Matters 2007, 2(2) 19) and others (Broderick et al., Chem. Mater. 2012, 24(10), 1786; Borodina et al., ACS Applied Mater. Interfaces 2009, 1(15) 996) for encapsulation of metal hydrides. Alternatively, uncharged polymers can be deposited on metal borohydrides using an interfacial polymer precipitation protocol that is induced by solvent evaporation (Borodina et al., J. Materials Chemistry 2010, 20(8), 1452). Thus, the mixed metal borohydrides can be formed into nanoparticles that can be encapsulated. The goal of the stabilization is both chemical, to reduce potential flammability of the materials and prevent degradation, and mechanical, to reinforce the material so that any structural changes during dehydrogenation are accommodated and volume expansion is minimized. By keeping the shell layer very thin compared to the size of the core, the impact on gravimetric and volumetric hydrogen density is minimized.

EXAMPLES

Example 1

Preparation of Li3Mg(BH4)5

(23) Mg(BH.sub.4).sub.2 was prepared from dibutylmagnesium and borane-methyl sulfide complex as described in the literature (Zanella, P.; et al. Inorg. Chem. 2007, 46, 9039-9041). A 0.5 M solution of Mg(BH.sub.4).sub.2 was prepared by dissolving 0.498 g of Mg(BH.sub.4).sub.2 (9.22 mmol) in 10 mL of anhydrous diethyl ether with stirring under Ar at room temperature. A commercial 0.5 M LiBH4 in diethyl ether (55.32 mL, 27.66 mmol) was added to the Mg(B1-14)2 solution using a canula. The resulting mixture was stirred overnight at room temperature. Solvent was removed in vacuo and 1.6 g of white product (Li.sub.3Mg(BH.sub.4).sub.5 complexed with residual solvent) was obtained after drying on a vacuum line overnight at room temperature. Upon thermolysis, the Li.sub.3Mg(BH.sub.4).sub.5 sample began to lose H.sub.2 at 166 C. and delivered 7.8 wt % H.sub.2 (measured) after heating to about 370 C.

Example 2

Preparation of Li2NaMg(BH4)5

(24) Mg(BH.sub.4).sub.2 is prepared from dibutylmagnesium and borane-methyl sulfide complex as described in the literature (Zanella, P.; et al. Inorg. Chem. 2007, 46, 9039-9041). A 0.5 M solution of Mg(BH.sub.4).sub.2 is prepared by dissolving 0.27 g (5.00 mmol) of Mg(BH.sub.4).sub.2 in 10 mL of anhydrous diethyl ether with stirring under Ar at room temperature. A commercial 0.5 M LiBH.sub.4 in diethyl ether (20 mL, 10.00 mmol) is combined with a solution of NaBH.sub.4 prepared by dissolving 0.19 g of NaBH.sub.4 in 10 mL of anhydrous 1,2-dimethoxyethane. The Mg(BH.sub.4).sub.2 solution is added to the LiBH.sub.4/NaBH.sub.4 solution using a syringe. The resulting mixture is stirred overnight at room temperature. The solvent is removed in vacuo and the resulting product (Li.sub.2NaMg(BH.sub.4).sub.5) is dried on a high vacuum line overnight. This material contains approximately 14.8 wt % H.sub.2 (theoretical).

Example 3

Preparation of Li4A13(BH4)13

(25) Al(BH.sub.4).sub.3 (1.42 g, 19.889 mmol) is prepared as described by Zhao, J-C, et al. (J. Phys. Chem. C. 2009, 113, 2) and dissolved in anhydrous diethyl ether as described by Bird and Wallbridge (J. Chem. Soc. 1965, 3923). A commercial 0.5 M LiBH.sub.4 in diethyl ether (53 mL, 26.52 mmol) is added to the Al(BH.sub.4).sub.3 solution by syringe at 78 C. The resulting mixture is allowed to warm slowly to room temperature and stirred for three hours under Ar. The solvent is removed in vacuo and the resulting product (Li.sub.4A1.sub.3(BH.sub.4).sub.13) is dried on a high vacuum line overnight. This material contains approximately 17.2 wt % H.sub.2 (theoretical).

Example 4

Preparation of Li3Ti(BH4)6

(26) Titanium borohydride (Ti(BH4)3) was prepared as the THF complex by reacting titanium (IV) isopropoxide (Aldrich, 2.22 mL, 0.008 mol) with borane-tetrahydrofuran (Aldrich, 1 M solution in THF, 33 mL, 0.033 mol) in 10 mL of anhydrous THF at 0 C. with stirring for two hours under argon atmosphere (Mirviss, S. B.; Dougherty, H. W.; Looney, R. W. U.S. Pat. No. 3,310,547 Mar. 21, 1967). Solvent and volatile by-products were removed in vacuo, first at 0 C. and then at room temperature. The deep blue oily product was then dissolved in 10 mL of anhydrous diethyl ether and cooled to 0 C. A solution of LiBH4 in diethyl ether was added (Aldrich, 0.5 M, 48 mL, 0.024 mol) and the mixture was warmed to room temperature with stirring under argon for 2 hours. A sticky aubergine-colored solid (2.1 g, Li3Ti(BH4)6 complexed with residual solvent) was obtained after solvent removal and drying on a vacuum line overnight. Upon thermolysis the product began to release H2 at 159 C. and delivered 7.3 wt % H2 (measured) after heating to 292 C.

Example 5

Preparation of LiCa(BH4)3

(27) Calcium borohydride bis-tetrahydrofuran (Aldrich, 2.14 g, 10 mmol) is dissolved in 10 mL of anhydrous diethyl ether and cooled to 0 C. with stirring under argon atmosphere. A solution of lithium borohydride (Aldrich, 0.5 M in diethyl ether, 20 mL, 10 mmol) is added and the mixture was allowed to warm to room temperature. After stirring for two hours the solvent is removed in vacuo to produce the mixed metal borohydride LiCa(BH.sub.4).sub.3, 13.1 wt % H.sub.2 (theoretical).

Example 6

Preparation of Li2NaCa(BH4)5

(28) Calcium borohydride bis-tetrahydrofuran (Aldrich, 2.14 g, 10 mmol) is dissolved in 10 mL of anhydrous diethyl ether and cooled to 0 C. with stirring under argon atmosphere. A solution of lithium borohydride (Aldrich, 0.5 M in diethyl ether, 40 mL, 20 mmol) and sodium borohydride (0.5 M in dimethoxyethyl ether, 20 mL, 10 mmol) is added and the mixture was allowed to warm to room temperature. After stirring for two hours the solvent is removed in vacuo first at room temperature and finally at 60 C. to produce the mixed metal borohydride Li.sub.2NaCa(BH.sub.4).sub.5, 13.2 wt % H.sub.2 (theoretical).

Example 7

Preparation of Li2NaTi(BH4)6

(29) Titanium borohydride (Ti(BH4)3) is prepared as the THF complex by reacting titanium (IV) isopropoxide (Aldrich, 2.22 mL, 7.5 mmol) with borane-tetrahydrofuran (Aldrich, 1 M solution in THF, 33 mL, 0.033 mol) in 10 mL of anhydrous THF at 0 C. with stirring for two hours under Argon atmosphere (Mirviss, S. B.; Dougherty, H. W.; Looney, R. W. U.S. Pat. No. 3,310,547 Mar. 21, 1967). Solvent and volatile by-products are removed in vacuo, first at 0 C. and then at room temperature. The deep blue oily product is then dissolved in 10 mL of anhydrous diethyl ether and cooled to 0 C. A solution of LiBH.sub.4 in diethyl ether is added (Aldrich, 0.5 M, 30 mL, 15 mmol), followed by 15 mL of a 0.5 M solution of sodium borohydride in dimethoxyethyl ether (Aldrich, 7.5 mmol) and the mixture was warmed to room temperature with stirring under argon. After stirring for two hours the solvent is removed in vacuo first at room temperature and finally at 60 C. to produce the mixed metal borohydride Li.sub.2NaTi(BH.sub.4).sub.6, 13.8 wt % H.sub.2 (theoretical).

Example 8

Preparation of LiAl(A1H4)(BH4)3

(30) Al(BH.sub.4).sub.3 (1.31 g, 18.271 mmol) is prepared as described by Zhao, J-C, et al. (J. Phys. Chem. C. 2009, 113, 2) and dissolved in 40 mL of anhydrous diethyl ether as described by Bird and Wallbridge (J. Chem. Soc. 1965, 3923). A commercial solution of 1.0 M LiA1H.sub.4 in diethyl ether (18.3 mL, 18.27 mmol) is added to the Al(BH.sub.4).sub.3 solution by syringe at 78 C. The resulting mixture is allowed to warm slowly to room temperature and stirred for three hours under argon. The solvent is removed in vacuo and the resulting product (LiAl(A1H.sub.4)(BH.sub.4).sub.3) is dried on a high vacuum line overnight. This material contains approximately 14.6 wt % H.sub.2 (theoretical).

Example 9

Preparation of LiMgA1(BH4)6

(31) Al(BH.sub.4).sub.3 (0.97 g, 13.579 mmol) is prepared as described by Zhao, J-C, et al. (J. Phys. Chem. C. 2009, 113, 2) and dissolved in 27 mL of anhydrous diethyl ether as described by Bird and Wallbridge (J. Chem. Soc. 1965, 3923). Mg(BH.sub.4).sub.2 is prepared from dibutylmagnesium and borane-methyl sulfide complex as described in the literature (Zanella, P.; et al. Inorg. Chem. 2007, 46, 9039-9041). A 0.5 M solution of Mg(BH.sub.4).sub.2 is prepared by dissolving 0.733 g (13.579 mmol) of Mg(BH.sub.4).sub.2 in 13.6 mL of anhydrous diethyl ether with stirring under Ar at room temperature. The Al(BH.sub.4).sub.3 and Mg(BH.sub.4).sub.2 solutions are combined at 78 C. with stirring under argon by canula transfer of the Mg(BH.sub.4).sub.2 solution into the Al(BH.sub.4).sub.3 solution. A commercial solution of 0.5 M LiBH.sub.4 in diethyl ether (27.2 mL, 27.16 mmol) is added to the Al(BH.sub.4).sub.3/Mg(BH.sub.4).sub.2 solution by syringe at 78 C. The resulting mixture is allowed to warm slowly to room temperature and stirred for six hours under Ar. The solvent is removed in vacuo and the resulting product (LiMgA1(BH.sub.4).sub.6) is dried on a high vacuum line overnight. This material contains approximately 16.3 wt % H.sub.2 (theoretical).

Example 10

Preparation of Li2Mg(BH4)4

(32) Mg(BH.sub.4).sub.2 was prepared from dibutylmagnesium and borane-methyl sulfide complex as described in the literature (Zanella, P.; et al. Inorg. Chem. 2007, 46, 9039-9041). A 0.5 M solution of Mg(BH.sub.4).sub.2 was prepared by dissolving 0.498 g of Mg(BH.sub.4).sub.2 (9.22 mmol) in 10 mLs of anhydrous diethyl ether with stirring under Ar at room temperature. A commercial 0.5 M LiBH.sub.4 in diethyl ether (36.86 mL, 18.43 mmol) was added to the Mg(BH.sub.4).sub.2 solution using a canula. The resulting mixture was stirred overnight at room temperature. Solvent was removed in vacuo and 1.23 g of white product (Li.sub.2Mg(BH.sub.4).sub.4 complexed with residual solvent) was obtained after drying on a vacuum line overnight at room temperature. Upon thermolysis Li.sub.2Mg(BH.sub.4).sub.2 began to evolve H.sub.2 at 130 C. and delivered 10.5 wt % H.sub.2 (measured) after heating to 375 C.

Example 11

Preparation of Lithium Based Mixed Metal Borohydrides

(33) The synthesis of many mixed metal borohydrides was performed using solutions of precursor compounds in an inert atmosphere at room temperature. Solutions of known concentrations of precursor compounds were prepared. Precursor solutions were mixed in desired stoichiometeries and deposited on hot plate array substrates using an automated liquid dispenser to produce libraries of the desired mixed metal borohydrides. Each hot plate substrate was a silicon microfabricated device, similar to those described by Guerin et al. (J. Comb. Chem. 2008, 10, 37-43), Amieiro-Fonesca et al. (Faraday Discuss. 2011, 151, 369-384), Anghel et al. (International Patent Publication No. WO 2009/101046 A1) and Guerin et al. (International Patent Publication No. WO 2005/035820 A 1). The precursors used were NaBH.sub.4, LiBH.sub.4, Mg(BH.sub.4).sub.2, Ca(BH.sub.4).sub.2.2THF and Ti(BH.sub.4).sub.3 in appropriate solvents (one or a combination of dimethylformamide (DMF), tetrahydrofuran (THF) and toluene). Arrays of the synthesized borohydrides were transferred to an ultra-high vacuum (UHV) chamber and degassed at approximately 310.sup.9 Torr) for approximately 10 hours. While in the UHV chamber, each of the synthesized borohydrides was tested using Temperature Programmed Desorption, heating each of the microhotplates sequentially, while monitoring the temperature and measuring the hydrogen partial pressure using a mass spectrometer.

(34) The compounds produced included the lithium based mixed metal borohydrides identified in FIG. 6. These mixed metal borohydrides did not react upon exposure to air and would release hydrogen gas within the 100 C. to 260 C. temperature range. As a comparison, the metal borohydride precursors used are shown in the lower portion of FIG. 6. Approximate onset and peak temperatures and gravimetric hydrogen capacities are shown. In general, the onset and peak temperatures for the mixed metal borohydrides in FIG. 6 were lower than those for the precursor compounds, and in most cases the gravimetric capacities were greater than the combined gravimetric capacities of the precursor compounds.

(35) Another inventive aspect is a hydrogen generator and a fuel cell system that uses the mixed metal borohydrides. The hydrogen generator is a hydrogen gas generating apparatus that produces hydrogen gas that is consumed by the fuel cell to produce electricity for an electronic device. The hydrogen generator includes a housing, a mixed metal borohydride as the reactant that will release hydrogen gas when heated, an ignition system including a heater to heat the reactant, and optionally a feed system. The reactant is contained in a solid composition that can be segregated into individual quantities of the composition. Quantities of the composition are referred to herein as pellets. As used herein, pellet means a mass of a solid composition that includes a reactant whose reaction is initiated by heating. The optional feed system is configured so individual pellets or groups of pellets are sequentially positioned in proximity to the heater, which can heat the pellets to initiate their thermal decomposition and evolve hydrogen gas. Alternatively, the ignition system can include a plurality of heaters for heating a plurality of stationary pellets.

(36) The pellets can be of any suitable size and shape. They can be sized and shaped to fit into the housing in a volume-efficient manner. For example, the pellets can be in the shape of round, oval or prismatic (e.g., trapezoidal, rectangular or square) pills, tablets, wafers, cakes or volumes of powder or granules. The pellet size and composition can be chosen to provide a desired quantity of hydrogen from each pellet, based on the size of the fuel cell stack and the power requirements of the electronic device, for example. The pellets can be formed in various ways. For example, they can be fed, poured, deposited (e.g., by coating, printing or otherwise applying), or formed (e.g., by molding or shaping) and secured (e.g., by adhering, fastening or the like) onto one or both surfaces of a carrier (e.g., in the form of a strip, ribbon, belt, sheet, string or the like). As used herein, strip is intended to include any such carrier configuration.

(37) The pellets contain at least one mixed metal borohydride reactant that releases hydrogen as the reactant decomposes by heating or otherwise. More than one reactant can be included.

(38) The pellets can also contain one or more additives. Examples of additives include binders (e.g., acrylates and styrene block copolymers), stabilizing compounds (e.g., solid bases), reaction accelerators (e.g., solid acids), catalysts as described above, ignition materials as described below, thermally conductive materials (e.g., metals, graphites and combinations and composites thereof), and so on.

(39) A pellet carrier strip should be sufficiently flexible to be fed by the feed system. The carrier including the pellets (i.e., the pellet strip) can be loaded into the housing in a rolled, folded or other configuration. In one embodiment a pellet strip is wound on a reel. More than one pellet strip can be disposed on a single reel, or one or more pellet strips can be disposed on separate reels. In another embodiment a pellet strip is folded in a Z-fold pattern (i.e., with alternating folds in opposite directions to create a stack of multiple layers of the pellet strip). The pellet strip can be disposed in a storage compartment within the housing, or the pellet strip can be disposed in a separate container that can be loaded into or attached to the housing. The hydrogen generator can be configured to contain one or more pellet strips, such as with at least one pellet strip in each of a plurality of compartments or containers, each of which can have a separate feed system. Pellets can be disposed on the carrier and the pellet strip can be disposed is such a manner as to facilitate feeding and provide a high density of pellets within the hydrogen generator. For example, the pellets can be disposed in one or more linear arrays along the carrier, or the pellet strip can be arranged so that pellets on one section of the carrier are nested between pellets on another section of the carrier.

(40) To prevent the transfer of heat from one pellet to adjacent pellets on the carrier, which could result in an uncontrolled initiation of the reaction of adjacent pellets, the carrier can be a material that is not a good conductor of heat. The carrier can be made from a material that does not react substantially during the thermal decomposition of the hydrogen containing reactant. This has the advantage of not generating any reaction products that might interfere with the functioning of the hydrogen generator or that would have to be removed from the hydrogen gas before being used by the fuel cell stack. Alternatively, the carrier can be made from a material that does react during the thermal decomposition of the hydrogen containing reactant, e.g., by burning. This can eliminate the need to collect and store the carrier after the pellets have been consumed. Examples of materials that can be suitable as carrier materials include polyimides such as KAPTONO from E.I. duPont de Nemours; polypropylene such as SCLAIR from Nova Chemicals (International) (Switzerland); TEFLON, TEFZEL, and MYLAR from E.I. duPont de Nemours; and paper.

(41) While it may be desirable to react more than one pellet at a time, in order to prevent the uncontrolled initiation of adjacent pellets, it is desirable for individual pellets or groups of pellets to be thermally insulated from one another. This can be accomplished in various ways, including the use of a carrier material that is a poor conductor of heat, spacing the pellets apart from one another, placing thermal insulation on the carrier between adjacent pellets, coating portions of the pellets with thermally insulating materials, and so on. Suitable thermal insulator materials include silica, silicon dioxide, silicon nitrides, silican carbide, glass, and polymers such as polyimides and epoxy-amine composites.

(42) A feed system feeds the pellet strip to sequentially position pellets, either individually or in groups, in proximity to the heater(s). Various types of feed systems can be used, such as augers, sprockets, ratchet wheels, and rotating belts. In one embodiment the feed system includes a sprocket. For example, teeth on the sprocket can engage or create perforations or indentations along the carrier to feed the pellet strip as the sprocket rotates (e.g., in a manner similar to that of a movie projector feeding film). In another example, the pellets and spaces between them function like the links of a chain that is driven by a sprocket. The feed system can include an indexing mechanism for indexing the pellet strip in increments. An example of an indexing mechanism is a ratchet, which will only allow movement of the drive mechanism in one direction. A ratchet may be mounted on a sprocket, for example.

(43) The ignition system heater heats one or more pellets positioned in proximity to the heater, resulting in a thermal decomposition reaction of the hydrogen containing reactant in the pellet(s). The ignition system can include more than one heater. Multiple heaters can be advantageous when a single heater does not produce sufficient heat, when more than one pellet is to be ignited at one time, when there are more than one stationary pellet, and when the hydrogen generator uses more than one pellet strip for example. Various types of heaters can be used. Examples include resistive heaters, infrared heaters, laser heaters, microwave heaters, semiconductor bridges, and so on.

(44) Alternatively, heating elements can be incorporated into the pellets or into the carrier. Electrical leads from the ignition system can make contact with heating element contacts so current to heat the heating elements is provided when the pellets are positioned in the desired location.

(45) One or more pellets are positioned in close enough proximity to the heater(s) for the heater(s) to heat the pellet(s) sufficiently that the hydrogen-containing reactant releases hydrogen gas. These proximal pellets may be spaced apart from the heater(s), or they may make contact with the heater(s).

(46) The heater can heat the hydrogen containing reactant directly, or it can heat an ignition material (a material that will react exothermally, producing the heat necessary for the thermal decomposition reaction of the hydrogen containing reactant). If the heater initiates reaction of the hydrogen containing reactant directly, the heater may provide heat only long enough to start the reaction, if the reaction is self-sustaining, or it may continue to provide heat for the entire reaction time. If an ignition material is used, the ignition material can be disposed within or in contact with a pellet, the ignition material can be a separate layer of the pellet (i.e., separate from a layer containing the hydrogen containing reactant), or the ignition material can be mixed with the hydrogen containing reactant.

(47) Examples of ignition materials (some of which can also contribute to the hydrogen yield) include thermite (Fe.sub.2O.sub.3 plus Al), iron powder plus KC1O.sub.4, TiH.sub.2 plus KC1O.sub.4, MnO.sub.2 plus LiA1H.sub.4, Ni plus Al, Zr plus PbCrO.sub.4, and LiA1H.sub.4 plus NH.sub.4C1.

(48) The hydrogen generator can include a waste zone for accumulating decomposing pellets, spent pellets and any residue (e.g., carrier material, ashes or other reaction or combustion byproducts) from the pellet strip. The waste zone can be separated from the pellet strip storage compartment by a wall. The wall can be a moving wall that defines a portion of the storage compartment. The wall can move as the pellet strip is consumed, thereby reducing the size of the storage compartment and increasing the size of the waste area. If the hydrogen generator includes more than one storage compartment, it can include a waste zone for the pellet strip in each compartment, or a single waste zone can be associated with more than one storage compartment. When the storage compartment is defined by a moveable wall, a portion of the feed system (e.g., a feed sprocket) can be moveable together with the moveable wall.

(49) Operation of the optional feed system, the ignition system or both can be controlled in various ways. A control system can be used. The control system can determine the need for hydrogen by monitoring the pressure within the fuel cell system, one or more electrical characteristics of the fuel cell stack, or one or more electrical characteristics of the electronic device, for example. The controller may communicate with the device or the fuel cell stack to determine when more hydrogen is needed. The control system can be completely or partially disposed in the hydrogen generator, the fuel cell stack, the electronic device being powered by the fuel cell stack, or any combination thereof. The control system can include a microprocessor or micro controller; digital, analog and/or hybrid circuitry; solid state and/or electromechanical switching devices; capacitors, sensing instrumentation, and so on.

(50) The housing of the hydrogen generator is made of a material that will withstand the heat and internal pressure that are produced to maintain desired dimensions and an adequate hydrogen seal. Examples of materials that may be suitable include metals such as aluminum and steel and polymeric materials such as polyphenylene sulfide and acrylonitrile butadiene styrene.

(51) The hydrogen generator can include various filters and/or purification units to remove undesired reaction byproducts and other contaminants from the hydrogen gas. The hydrogen gas can follow a pathway within the hydrogen generator after being released from the mixed metal borohydride fuel source. The fillers and/or purification units can be contained within this pathway or external to the hydrogen generator.

(52) The hydrogen generator can also include various fittings, valves, and electrical connections for providing hydrogen to and interfacing with the fuel cell and/or an electrical appliance being provided with power by the fuel cell system.

(53) The hydrogen generator can include various safety features such as a pressure relief vent to release excessive pressure and a mechanism to stop the feeding of pellets to the ignition system if the internal temperature exceeds an established limit.

(54) The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawing and described above is merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.