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Modified Lithium Borohydrides for Reversible Hydrogen Storage (2) Ming Au,* Arthur Jurgensen, and Kristine Zeigler SaVannah RiVer National Laboratory, Aiken, South Carolina 29808, USA ReceiVed: August 24, 2006; In Final Form: October 12, 2006
This paper reports the results of the effort to destabilize lithium borohydride for reversible hydrogen storage. Various metals, metal hydrides, and metal chlorides were selected and evaluated as destabilization agents for reducing dehydriding temperatures and improving dehydriding/rehydriding reversibility. The most effective material was LiBH4 + 0.2MgCl2 + 0.1TiCl3 which starts desorbing 5 wt % of hydrogen at 60 °C and can be rehydrogenated to 4.5 wt % at 600 °C and 70 bar. X-ray diffraction and Raman spectroscopic analysis show the interaction of LiBH4 with additives and the unusual change of B-H stretching.
1. Introduction To find reversible hydrogen storage materials with sufficient specific energy and energy density is a grand challenge in migrating to a hydrogen economy. The high gravimetric (18.4 wt %) and volumetric (121 kg/m3) hydrogen capacity of lithium borohydride motivates the high interest in developing this material as an effective hydrogen storage media. The main focus of lithium borohydride research is to reduce the dehydrogenation temperature and to make the material hydrogen rechargeable at moderate pressures and temperatures. In our previous work, several oxides had been found to be effective in reducing the dehydrogenation temperature of LiBH4 from 400 to 200 °C. These modified LiBH4-based materials show ∼8-9 wt % of reversible hydrogen storage capacity at 600 °C and 70 bar in limited dehydriding/rehydriding cycles.1 Although this is an encouraging step forward, the elevated rehydrogenation conditions are still impractical for on-board hydrogen storage applications. It is believed that the oxides may play a role as decomposition promoters (or catalysts) for releasing hydrogen at lower temperatures. A fundamental change in the thermodynamic stability of LiBH4 will be required for releasing hydrogen at lower temperatures, for example, less than 100 °C. It is known that the electrons in ionic compounds such as LiBH4 and NaBH4 are strongly localized, that is, highly ionic, which gives these materials their stable thermodynamics.2,3 Partial substitution of the cation (the Li+1 in LiBH4) with other cations may change the borohydride bond structure, hopefully weakening B-H bond strengths. It is assumed that Li substitution with less metallic cations might reduce the ionic character of the M+1(metal)-[BH4]-1 system and delocalize its electrons resulting in a lower stability. In other words, the thermodynamic stability of the metal borohydrides MaBH4 can be reduced by partial substitution of the metal element Ma with the element Mb where Mb is less metallic in nature than Ma. By carefully selecting Mb, the B-H bonds may be weakened, and the enthalpy (∆H) of decomposition of the metal borohydride might be reduced, resulting in a lower dehydrogenation temperature. * Corresponding author. Tel.: 803-507-8547. Fax: 803-652-8137. Email:
[email protected].
The substitution concept can be expressed by the following dehydrogenation reactions:
unmodified borohydrides 3 MaBH4 S MH + B + H2 2
∆Ha
partially substituted borohydrides 3 a a Mbx BH4 S M1-x Mbx H + B + H2 M1-x 2
∆Hab
expectation ∆Hab < ∆Ha However, the partial substitution of the lithium borohydride has no experimental data reported to date. The other opportunity to reduce the dehydrogenation temperature of the lithium borohydride is using the additives to change the reaction from the irreversible thermal decomposition at high temperature to the reversible ion-exchange interaction with hydrogen liberation at low temperature. For example:
hydrogen releasing interaction LiBH4 + MX S LiX + MB + 2H2
∆Hc
expectation ∆Hc < ∆Ha There is a report showing that the interaction of LiBH4 + MgH2 f LiH + MgB2 + H2 is reversible and releases 8 wt % of hydrogen at 450 °C.4 However, the destabilizing thermodynamic stability either by substitution or by additive interaction is at the price of a reduction in hydrogen storage capacity. The best compromise has to be made between lower operating temperature and sufficient storage capacity. In this exploratory work, the research was focused on further reducing the dehydriding temperature and moderating the rehydrogenation conditions. Specially designed formulations
10.1021/jp065490h CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2006
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with selected additives and processes have been developed for the evaluation of their hydrogen storage properties.5 The experiment results are presented in this paper. 2. Experimental Details The LiBH4 powder (99.99% purity) and the additives such as metals, metal hydrides, and metal chlorides (99.9-99.99% purity) were purchased from Sigma-Aldrich and used directly without any pretreatment. Several metals, metal hydrides, and metal chlorides were selected and added to LiBH4 as potential substitution or interaction agents. For fast screening of dehydrogenation temperature and capacity, ball milling was employed for preparation of the samples of new materials.6 Two grams of the appropriate LiBH4/ additives mixture was placed in a 25 mL hardened steel grinding bowl with three 11 mm diameter tungsten carbide balls within an argon glovebox. The sealed grinding bowls were taken out of the glovebox, put on a Frisch-7 planetary ball mill, and ground for 10 h at 600 rpm. The materials were then taken out of the grinding bowls and loaded into a stainless steel reactor. Then, the reactor was connected with a semi-automatic Sieverts apparatus. After evacuation to less than 5 mbar, the reactor connected with a chamber of known volume was heated from ambient temperature to 600 °C at a 5 °C/min heating rate. A vacuum transducer registered the pressure increase in the chamber, and then a data acquisition system converted it to hydrogen desorption. This method of measurement is often referred to as temperature programmed desorption (TPD). A mechano-thermal diffusion process (MTDP) was developed to synthesize new materials. The mixture of LiBH4/additives was placed in a 25 mL hardened steel grinding bowl with three 11 mm diameter tungsten carbide balls in an argon glovebox. The sealed grinding bowls were taken out of the glovebox, put on a Frisch-7 planetary ball mill, and ground for 1 h at 600 rpm. The powder mixtures were transferred to a stainless steel reactor for sintering at 350 °C and 100 bar of hydrogen for 8 h. Afterward, the sintered materials were crushed and ball milled with the catalyst for 10 h at 600 rpm.7 The MTDP was developed to perform the possible partial elemental substitution and additive interaction in an attempt to achieve a lower dehydriding temperature and improve reversibility. The new materials were evaluated for their dehydriding/rehydriding properties and hydrogen storage capacity. On the basis of the screening results, the materials LiBH4 + 0.2MgCl2 + 0.1TiCl3, LiBH4 + 0.3MgCl2 + 0.1TiCl3, and LiBH4 + 0.076MgCl2 + 0.047TiCl3 were synthesized by MTDP. The formulations of the materials reported in this paper are given in molar ratios. All of the materials’ transferring and handling were conducted in an argon glovebox. To assess the potential for engineering applications, we performed isothermal hydrogen desorption and absorption measurements in the Sieverts apparatus. Selected destabilized borohydrides were investigated by X-ray diffraction (XRD) analysis and Raman spectroscopy (Raman) to determine the changes in their phase composition and H-B binding features. The hydrogen purity from thermal decomposition of the unmodified LiBH4 was measured by a mass spectrometer in our previous work. About 10 ppm of BH4 was detected.1 It is expected that the modified LiBH4 will generate no more BH4 than 10 ppm during dehydrogenation. We will report the results of our measurement when new data are available. 3. Results and Discussion 3.1. TPD Screening of the Modified LiBH4-Based Materials. In this investigation, 14 modified LiBH4-based materials
Figure 1. TPD of metal-modified LiBH4-based materials.
TABLE 1: Modified LiBH4-Based Materials preliminary result sample
composition
positive
1 2 3 4 5 6 7 8 9 10 11 12 13 14
LiBH4 + 0.2 Mg LiBH4 + 0.2MgCl2 + 0.1TiCl3 LiBH4 + 0.076MgCl2 + 0.047TiCl3 LiBH4 + 0.5MgH2 + 2%TiCl3 LiBH4 + 0.5NaH LiBH4 + 0.5NaH + 0.1TiO2 LiBH4 + 0.2C LiBH4 + 0.2C + 0.01TiCl3 LiBH4 + 0.04Ni LiBH4 + 0.2Al 0.95LiBH4 + 0.05Ca 0.95LiBH4 + 0.05In LiBH4 + 0.1Al + 0.05TiO2 LiBH4 + 0.5CaH2
x x x x
negative
x x x x x x x x x x
have been synthesized (Table 1) by ball milling and evaluated using TPD. Mg, Al, MgCl2, MgH2, CaH2, and TiCl3 show the positive effect on reduction of dehydrogenation temperature. But, other additives such as NaH, Ni, Ca, In, and graphite exhibit a negative influence. The results are listed in Table 1. 3.1.1. Metal Modified LiBH4-Based Materials. Commercial LiBH4 decomposes at a slow rate starting at approximately 400 °C. At about 450 °C, the decomposition rate appreciably accelerates, resulting in a final release of 9 wt % of hydrogen at 600 °C. In this investigation, LiBH4 was modified by ball milling with the metals Mg, Al, Ca, In, and carbon graphite. The TPD results show that Mg and Al have a positive influence on the reduction of the dehydriding temperature (lower stability), but Ca, In, Ni, and carbon graphite exhibit a negative effect (see Figure 1). The material LiBH4 + 0.2Mg liberated 1 wt % of hydrogen at a very slow rate starting from 60 °C. The dehydriding accelerated when the temperature increased above 300 °C, and the material desorbed 9 wt % of hydrogen up to 600 °C. The material LiBH4 + 0.2Al desorbed 0.2 wt % of hydrogen slowly at 80 °C and 7.8 wt % of hydrogen rapidly from 300 to 600 °C. It is interesting that there are two plateaus, one at 400 °C and one at 500 °C, in the TPD curve of the aluminum-doped LiBH4. This may be indicative of the formation of new compounds, the products of the interaction of LiBH4 with Al. These compounds may be less stable or more stable than LiBH4. It is our hope to find less stable intermediate compounds with low desorption temperatures and dehydriding/ rehydriding reversibility. To investigate the benefit of Al doping, the material LiBH4 + 0.2Al was rehydrided at 100 bar and 600 °C after the first
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Figure 2. Rapid dehydriding of LiBH4 + 0.2Al material. Figure 4. Hydrogen desorption of the metal-hydride-modified LiBH4 materials.
Figure 3. TPD of LiBH4-based materials modified by MgCl2 and TiCl3.
desorption at 600 °C and 5 mbar. The second desorption was then carried out. As Figure 2 shows, the first plateau disappears in the second TPD; the desorption starting temperature has gone up from 80 to 350 °C, and the capacity has decreased from 8 wt % to 3.5 wt %. It appears that the Al-doped LiBH4 lost some of its constituents during the first dehydriding resulting in the disappearance of the first plateau and a decrease of hydrogen storage capacity. It is expected that the reversibility of dehydriding/rehydriding will fade in further cycling. The additives, Ca, In, and graphite, show no influence or a negative influence on the reduction of the hydrogen desorption temperature. They also did not generate the plateau in TPD curves (Figure 1). It appears that the lithium borohydride has no interaction with the metals Ca, In, and carbon. 3.1.2. Metal-Chloride Modified LiBH4-Based Materials. In this investigation, three LiBH4-based materials modified by the addition of MgCl2 and TiCl3 at different molar ratios have been synthesized. Their TPD results are shown in Figure 3. The material LiBH4 + 0.2 MgCl2 + 0.1TiCl3 released 5.5 wt % of hydrogen starting from 60 °C, which is the lowest dehydrogenation temperature ever reported for lithium borohydrides. Unlike the metal-oxide-modified LiBH4 that has a typical slow dehydrogenation rate below 350 °C,1 the material LiBH4 + 0.2MgCl2 + 0.1TiCl3 desorbed 5.2 wt % of hydrogen linearly at a high rate from 60 to 400 °C. Obviously, the additives MgCl2 and TiCl3 have made LiBH4 much less stable. The additives also changed the shapes of the TPD curves from sigmoidal, typically shown for oxide-modified LiBH4 and unmodified LiBH4, to nearly linear. This implies a different reaction mechanism. As the LiBH4 + 0.3MgCl2 + 0.1TiCl3 shows,
increasing additive loading did little in reducing the dehydrogenation temperature further but decreased the hydrogen storage capacity by about 0.9 wt %. As shown in Figure 3 for LiBH4 + 0.076MgCl2 + 0.047TiCl3, the reduction of additive loading resulted in a slow reaction and a capacity increase. The shape of the TPD curve also changed back from linear to sigmoidal which implied that there was a change in the reaction mechanism. It is encouraging to see that most of the dehydrogenation reaction was completed below 400 °C, although the capacity is low. It is believed that the additives MgCl2 and TiCl3 may reduce the thermodynamic stability of LiBH4 through interactions or possible, but unlikely, elemental substitution. The possible interactions, their products, and the contribution to a lower dehydrogenation temperature will be discussed in the following XRD data analysis section. The overall low capacity of MgCl2- and TiCl3-doped LiBH4 is attributed to heavy additive loading because the MgCl2 and TiCl3 are counted as 67.4 wt % of the total weight of the material. It is our hope that optimization of the additive loading amount will increase the dehydriding capacity while retaining a lower dehydriding temperature. The material LiBH4 + 0.3MgCl2 + 0.1TiCl3 was selected for the evaluation of isothermal desorption and absorption. The results will be presented later in section 3.2. 3.1.3. Metal-Hydride Modified LiBH4-Based Materials. In an attempt to form less-stable compounds, several metal hydrides such as MgH2, NaH, and CaH2 were added to LiBH4 using the ball-milling process. The TPD results are shown in Figures 4 and 5. The material LiBH4 + 0.5MgH2 + 0.02TiCl3 desorbed 8.5 wt % of hydrogen at 600 °C with two stages in the desorption curve. The first desorption released about 2.8 wt % of hydrogen slowly from 100 to 400 °C, and the second one librated 5.8 wt % from 400 to 600 °C with a relatively fast desorption rate. The first stage may be attributed to MgH2 dehydriding, and the second one may correspond to LiBH4 dehydriding. The possible interaction of LiBH4 and MgH2 may take place during ball milling and following dehydriding.4 The material LiBH4 + 0.5CaH2 desorbed hydrogen starting from 50 °C with 1.5 wt % of hydrogen evolved at 400 °C and 8.2 wt % of hydrogen evolved at 500 °C. It demonstrates that CaH2 promotes hydrogen liberation from LiBH4 through interaction. The interaction of CaH2 with organic compounds through the ball-milling or mechano-chemical process was reported by Cao et al.8 Although most of the additives selected in this investigation have a positive effect on dehydrogenation of LiBH4, some
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Figure 6. Isothermal hydrogen desorption of the material LiBH4 + 0.2MgCl2 + 0.1TiCl3. Figure 5. Hydrogen desorption of the modified LiBH4 materials with negative additives.
additives do show a negative impact on the dehydrogenation of LiBH4, as Figure 5 illustrates. One possible explanation is that the LiBH4 may be transformed to a more stable configuration through an ion-exchange reaction with the additives. For example, the Na ion in NaH may substitute for the Li ion in LiBH4 to form a more stable borohydride, NaBH4, in the materials LiBH4 + 0.5NaH and LiBH4 + 0.5NaH + 0.1TiO2. The materials LiBH4 + 0.04Ni and LiBH4 + 0.05In also show negative results but are not listed in the chart. The material 0.95LiBH4 + 0.05Ca shows a negative effect when adding Ca metal (Figure 1). However, the material LiBH4 + 0.5CaH2 demonstrated the positive role of CaH2 as an additive in decomposition temperature reduction (Figure 4). It is believed that the Ca metal is covered by a CaO layer preventing Ca from interacting with LiBH4. In contrast, CaH2 provides the fresh metallic Ca through decomposition during ball milling. In summary, the additives Mg, MgCl2, CaH2, and TiCl3 reduced dehydriding temperatures from 350 to 100 °C or lower. The dehydrogenation reactions were completed in a single stage. The positive effect of these additives implies that the thermodynamic stability of the LiBH4 was reduced, possibly because of the ion-exchange reaction or partial Li substitution by Mg and Ca. Ongoing material characterization will provide the evidence to confirm or denounce the cation substitution and additive interaction theories. The additives MgH2 and CaH2 also reduced dehydriding temperatures and produced more hydrogen at a lower temperature. The dehydrogenations were completed in two stages, at low and high temperature. Somehow, the two stages may be dominated by the decomposition of the hydride additives in the lower temperature ranges and by the LiBH4 in the higher temperature ranges. However, the interactions of the additives with LiBH4 should be considered because of their important role. Some of the additives such as NaH, Ni, In, Ca, Al, and graphite have a negative effect by increasing the dehydriding temperature of LiBH4. One of the possible reasons is the formation of more stable metal borohydrides. The MTDP may promote an interaction and produce interesting new compounds that may play a role that facilitates the reversible reaction at moderate conditions. 3.2. Isothermal Dehydrogenation of the Selected Modified Borohydrides. Although TPDs provide a picture of the dehydrogenation process in a broad temperature range, isothermal dehydrogenation studies are normally used to judge material performance such as its storage capacity and kinetics at constant temperature. The isothermal dehydrogenation of the material
Figure 7. Hydrogen absorption of the material LiBH4 + 0.2MgCl2 + 0.1TiCl3.
LiBH4 + 0.2MgCl2 + 0.1TiCl3 synthesized with MTDP is shown in Figure 6. At 400 °C, the material desorbed 2.5 wt % of hydrogen rapidly in the first 15 min and 4.9 wt % of hydrogen following two incubation periods in 18 h. The second desorption at 500 °C was almost the same as the first with 4.9 wt % of hydrogen released in 18 h. In the third dehydriding, the materials desorbed 4.25 wt % of hydrogen in 22 h. It is interesting to note that there is no incubation during the third dehydriding. It may imply the completion of the additives’ interactions with thermal cycling. Repeated dehydriding and rehydriding shows some reversibility. However, the desorption kinetics are too slow for practical applications, and the additives reduced the hydrogen capacity. It is possible that some percentage of hydrogen can be released during additive interaction with LiBH4. 3.3. Isothermal Rehydriding of the Selected Borohydrides. After dehydriding at 600 °C for 1 h, LiBH4 + 0.2MgCl2 + 0.1TiO2 was selected for rehydrogenation at 600 °C and 70 bar. As Figure 7 shows, the material absorbs 2.8 wt % of hydrogen in 17 h, but it absorbed more hydrogen (4.4 wt %) in the second rehydrogenation. It is not clear if more dehydriding/rehydriding cycling will result in a higher capacity or faster kinetics. But, it demonstrates that the material is reversible in the limited cycling. On the other hand, the rehydrogenation kinetics of the destabilized LiBH4 is much slower compared with oxidecatalyzed LiBH4 reported in the previous paper.1 The rehydriding temperature and pressure are still too high for immediate application. The development of reversible borohydrides requires more systematic experiments and fundamental studies for understanding the nature of the destabilization, either element substitution or additive interaction.
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Figure 8. XRD spectra of the material LiBH4 + 0.2MgCl2 + 0.1TiCl3.
Figure 9. Raman spectra of LiBH4 + 0.2MgCl2 + 0.1TiCl3 and LiBH4 + 0.3MgCl3 + 0.1TiCl3.
3.4. XRD Analysis of the Material LiBH4 + 0.2MgCl2 + 0.1TiCl3. In an attempt to understand the reactions and their products, we analyzed samples of LiBH4 + 0.2MgCl2 + 0.1TiCl3, synthesized, dehydrided, and rehydrided, by XRD. The results are presented in Figure 8. After MTDP, LiCl peaks dominate the XRD spectrum with a few small peaks of MgO that may be generated by oxygen introduced during sample preparation. A small peak of TiCl3 (33.8° 2θ) appears on the shoulder of the LiCl peak at 34° 2θ. The large peak at 25.5° 2θ comes from the polyethylene film that is used to isolate the sample from air. Interestingly, LiBH4 almost disappeared from the spectrum. The possible reasons could be (1) interactions of LiBH4 with MgCl2 and TiCl3 resulting in its decomposition even before heating, (2) LiBH4 transforming to an amorphous state, or (3) both of the above. There are a couple of small negligible peaks at 24° 2θ and 27° 2θ which may be the residual LiBH4 that did not react with the additives. It was observed in our previous research that LiBH4 always had very low peak intensity in the modified LiBH4-based material XRD spectrum. Hoekstra and Katz confirmed the interactions of 2TiCl4 + 8LiBH4 f 2Ti(BH4)3 + 8LiCl + B2H6 + H2 and 2TiCl4 + 3Al(BH4)3 f 2TiCl(BH4)2 + 3AlCl2BH4 + B2H6 + H2 by analyzing the reaction products.9 It is reported that the dehydrogenation temperatures of Ti(BH4)3 and Mg(BH4)2 are 25 °C
and 260-280 °C, respectively.9,10 At room temperature and ambient pressure, Ti(BH4)3 decomposed slowly within 160 h by releasing hydrogen and a trace amount of B2H6 gas.9 There is a possibility that similar ion-exchange interactions of 3LiBH4 + TiCl3 f 3LiCl + Ti(BH4)3 and 2LiBH4 + MgCl2 f 2LiCl + Mg(BH4)2 might take place in our materials during dehydriding. However, we cannot confirm if the very low dehydriding temperature (60 °C) should be attributed to the decomposition of Ti(BH4)3 because neither Ti(BH4)3 nor Mg(BH4)2 can be identified in the XRD spectrum. After dehydrogenation, the XRD spectrum showed peaks of LiCl, MgO, and TiCl3. It appears that the TiCl3 reacts with LiBH4 but not completely. In Hoekstra and Katz’s early work, the metallic borides such as TiB3 and TiB2 were observed after Ti(BH4)3 released hydrogen.9 However, we did not observe any metallic deposition on our reactor. The whole material was sintered as a porous dark green block. We did not analyze the composition of the hydrogen stream, but the formation of borane and TiB2 were detected during LiBH4 dehydrogenation in our previous work.1 In that paper, we emphasized that it is crucial to prevent any boride gases such as BH3 and B2H6 from forming to ensure the reversibility of dehydriding/rehydriding reactions. We will report the influence of the additives on the prevention of the formation of borane when new data are available. The XRD spectrum of the rehydrided sample shows LiCl and LiBH4 only. The LiBH4 peak intensities are not proportional to their concentration in the materials. It is possible that some compounds are in the undetectable amorphous state. MgO is likely reduced by hydrogen. The consistent appearance of LiCl in the three samples and disappearance of TiCl3 in the rehydrided sample imply that TiCl3 and MgCl2 interacted with LiBH4 producing new compounds. The XRD data alone are not sufficient to give a rational elucidation of these complicated but interesting interactions. A fundamental study on thermal decomposition and rehydrogenation processes has been proposed for future research. 3.5. Raman Scattering Analysis of the Modified LiBH4 Materials. To find clues why the additives can reduce the dehydriding temperature, we have investigated the materials showing a lower dehydriding temperature and limited revers-
Modified Lithium Borohydrides ibility by Raman. At room temperature, the commercial LiBH4 shows two Raman spectroscopy active internal BH4-1 vibrations, ν4 and ν′4, at 1253 and 1287 cm-1 and two overtones, 2ν4 and 2ν′4, at 2240 and 2274 cm-1, respectively, as shown by the green spectrum in Figure 9. However, the ν4, ν′4, and 2ν4 stretching disappear from the spectrum of the material LiBH4 + 0.2MgCl2 + 0.1TiCl3. The 2ν′4 stretching is weakened and shifted to 2300 cm-1 as the blue spectrum shows. Because of insufficient data, we cannot confirm if the change of the B-H bond is the result of partial substitution or additive interaction. A similar spectrum (red) was obtained from material LiBH4 + 0.3MgCl2 + 0.1TiCl3. 4. Conclusions In an attempt to destabilize lithium borohydride, various metals, metal hydrides, and metal chlorides have been selected and evaluated as possible destabilization agents. The experimental results show that additives, such as Mg, Al, MgH2, CaH2, TiCl3, and MgCl2, are effective in reducing the dehydriding temperature, but some additives have a negative effect such as Ni, C, In, Ca, and NaH. The destabilized lithium borohydrides are reversible in the limited dehydriding/rehydriding cycles with slow reaction kinetics. The required rehydriding temperature and pressure are still elevated. The lowest dehydriding starting temperature was 60 °C for the material LiBH4 + 0.2MgCl2 + 0.1TiCl3. It desorbed 5 wt % of hydrogen at 400 °C and absorbed 4.5 wt % of hydrogen at 600 °C and 70 bar during dehydriding/rehydriding cycles. The XRD of the material LiBH4 + 0.2MgCl2 + 0.1TiCl3 shows that the additive MgCl2
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26487 and TiCl3 interacted with LiBH4 to form LiCl and other compounds resulting in a lower dehydrogenation temperature. Raman spectra analysis shows the H-B stretch changing after adding MgCl2 and TiCl3 into LiBH4. It is not our intention to interpret the complicated reactions using limited data. However, understanding the fundamental aspects of the additive modification and identifying the intermediate compounds during dehydriding and rehydriding will pave the way for developing new hydrogen storage materials with desirable performance at practical operating conditions. Acknowledgment. The author thanks Drs. J. Holder, T. Motyka, P. Cloessner, L. Heung, and K. Shanahan for comments and encouragement. This project was financially supported by the National Nuclear Security Administration PDRD Program. Savannah River National Laboratory is operated by Washington Savannah River Company for the U.S. Department of Energy under Contract No. DE-AC09-96SR18500. References and Notes (1) Au, M.; Jurgensen, A. J. Phy. Chem. B. 2006, 110, 7062-7067. (2) Zuttel, A.; Rentsch, S.; Fischer, P.; Wenger, P.; Sudan, P.; Mauron, Ph.; Emmenegger, Ch. J. Alloys Compd. 2003, 356-357, 515-520. (3) Nakamori, Y.; Orimo, S. J. Alloys Compd. 2004, 370, 271-275. (4) Vajo, J.; Skeith, S. J. Phys. Chem. B. 2005, 109, 3719-3722. (5) Au, M. U.S. Patent Application 2006/0046930, 2006. (6) Au, M. U.S. Patent Application 2006/0194695, 2006. (7) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1-184. (8) Cao, G.; Cocco, G.; Doppiu, S.; Monagheddu, M.; Orru, R.; Sannia, M. Ind. Eng. Chem. Res. 1999, 38, 3218-3224. (9) Hoekstra, H. R.; Katz, J. J. J. Am. Chem. Soc. 1949, 71, 24882492. (10) Wiberg, E.; Bauer, R. Z. Naturforsch. 1950, 5b, 397.
