LiBH4 Systems for

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Reaction Mechanisms in the Li3AlH6/LiBH4 and Al/LiBH4 Systems for Reversible Hydrogen Storage. Part 1: H Capacity and Role of Al Young Joon Choi,† Jun Lu,‡ Hong Yong Sohn,* and Zhigang Zak Fang Department of Metallurgical Engineering, University of Utah, 135 South 1460 East Room 412, Salt Lake City, Utah 84112, United States ABSTRACT: Lithium-based complex hydrides, including lithium aluminum hydrides (LiAlH4, Li3AlH6) and lithium borohydride (LiBH4), are some of the most attractive materials for hydrogen storage due to their high hydrogen contents. In the present work, we investigated the hydrogen storage properties of combined systems of Li3AlH6/LiBH4 and Al/LiBH4, both of which exhibit favorable storage properties owing to the formation of AlB2 during dehydrogenation. TGA data showed that TiCl3-doped Li3AlH6/2LiBH4 and 0.5Al/LiBH4 release ∼8.8 and ∼8.4 wt % H2, respectively, with ∼3.8 and ∼5.8 wt % release after rehydrogenation of the dehydrogenation product. XRD results identified LiH and AlB2 phases in the dehydrogenated products, which suggests a mechanism by which Al contributes to the remarkable improvement of the reversible storage properties of LiBH4 in terms of temperature and pressure for H2 release and uptake.

1. INTRODUCTION In recent years, the growing demand for efficient and clean alternative fuels has increased the attention to the development of hydrogen storage materials. However, one of the serious obstacles to the use of hydrogen as a fuel, especially in automotive applications, remains the lack of practical methods to store it. Although substantial progress has been made in the past few years in the discovery of new materials, none of them have demonstrated a sufficient reversible storage capacity in terms of the gravimetric and volumetric contents of hydrogen required for practical applications. A great deal of research and development work on hydrogen storage materials has focused on solid metal hydrides. Typically, these materials are complex light metal hydrides of three categories, alanates,1-15 borohydrides,16-33 and amides34-39 such as sodium alanate (NaAlH4), lithium borohydride (LiBH4), and lithium amide (LiNH2). The primary driving force behind the interest in these complex metal hydrides compared with simple metal hydrides such as LiH and MgH2 is that the latter are too stable at temperatures < 300 °C.40 There are also a number of other critical requirements including sufficiently high rates of dehydrogenation and hydrogenation reactions and reasonably low temperature and pressure for the rehydrogenation reactions. The properties and performances of the complex metal hydrides fall short of meeting one or more of these critical requirements. LiAlH4 has attracted much interest for on-board application due to its high inherent hydrogen capacity (∼10.6 wt %). During decomposition at elevated temperatures, LiAlH4 first forms an intermediate compound Li3AlH6 and then LiH at around 160210 °C, liberating 5.3 and 2.6 wt % of hydrogen, respectively.10-15 The dehydrogenation of LiH occurs at a much higher r 2011 American Chemical Society

temperature, at around 720 °C,40 releasing another 2.6 wt % of hydrogen. However, this temperature is too high for practical vehicular applications.40 In addition, the dehydrogenation reaction of LiAlH4 is not easily reversed,5,7,14,15 which is a critical requirement for on-board applications. Recently, using the CALPHAD thermodynamic modeling tool, Jang et al.14 found that the reaction from Li3AlH6 to LiAlH4 (Li3AlH6 þ 3H2 þ 2Al = 3LiAlH4) requires more than 103 bar of hydrogen pressure above room temperature. This was confirmed by Zaluski et al.3 and Balema et al.4,6 using differential thermal analysis (DTA). On the other hand, the results of Jang et al.14 have shown that the reaction 3LiH þ Al þ 1.5H2 = Li3AlH6 is thermodynamically feasible. Lithium borohydride has recently received attention because of its high theoretical storage capacity (18.5 wt %). Unfortunately, the potential for practical use of LiBH4 is severely limited by the slow kinetics of release at a moderate temperature. Also, the rehydrogenation conditions, 600 °C at 150 bar, are impractical for on-board application.21,23 Partial decomposition of LiBH4, which liberates 13.7 wt % H2 according to the following reaction 1, has a standard reaction enthalpy (ΔH) of ∼67 kJ/mol H2, translating to a dehydrogenation temperature of 410 °C at 1 bar on the basis of the Van’t Hoff equation.19 It is reported that the appearance of Li2B12H12, which is a well-known reaction intermediate of LiBH4, is also observed during reaction 1.31,32,41 LiBH4 ¼ LiH þ B þ 1:5H2 ð>410 °CÞ ΔH ¼ þ 67 kJ=mol H2

ð1Þ

Received: September 23, 2010 Revised: January 13, 2011 Published: March 10, 2011 6040

dx.doi.org/10.1021/jp109112t | J. Phys. Chem. C 2011, 115, 6040–6047

The Journal of Physical Chemistry C

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Therefore, many researchers have focused on destabilizing it to lower its dehydrogenation temperature by the use of additives such as metals,23,26,28,30,33 metal halides,21,24 oxides,16,21 sulfides,24 hydrides,19,23,28 and, more recently, nanoporous scaffolds.24 One of the notable results was reported by Vajo et al.,19 who studied the destabilization of LiBH4 using MgH2 as an additive. Their results indicated that the reaction between LiBH4 and MgH2 reversibly released over 10 wt % of hydrogen with a reaction enthalpy that is 25 kJ/mol H2 lower than that of pure LiBH4, according to reaction 1. However, it took up to 100 h to attain a plateau pressure of 4.5 bar at 315 °C for the dehydrogenation, and the temperature range for the reaction was high (up to 500 °C) even when catalyzed. More recently, several research groups have demonstrated that calculations based on the first-principles-based density functional theory (DFT) can provide a computational screening for thermodynamically suitable destabilizing agents among a large number of candidates.22,25,27 These are encouraging results that have motivated many research groups to investigate several promising reactions, especially the formation of stable metal boride phases such as MgB2, AlB2, and CaB6 from LiBH4. In particular, LiBH4, which is reversible only at high temperatures and pressures, can be made reversible at lower temperatures by adding MgH2 or Al, which decreases the ΔH of reaction and improves reaction kinetics.19,24,26,28-30,33 On the basis of the above information, we anticipated that a new combined system of LiBH4 and Li3AlH6 in a 2:1 ratio, which gives a composition for the thermodynamically preferred boride formation, would be reversible. The overall reaction equation of the combined system would be given by Li3 AlH6 þ 2LiBH4 ¼ 5LiH þ AlB2 þ 4:5H2

ð2Þ

In this paper, we describe the synthesis and characterization of the Li3AlH6/LiBH4 and the role of Al on the hydrogen release/ uptake reactions for the first time. The role of Al was further studied by testing the Al/LiBH4 system. Possible reaction mechanisms and paths of the combined systems will also be discussed. A further examination of the reaction paths by NMR analyses of the reaction samples is presented in the accompanying Part 2.41

2. EXPERIMENTAL APPARATUS AND PROCEDURE The starting materials, lithium borohydride (LiBH4, 90%), titanium chloride (TiCl3, 99.999%), aluminum (99.5%) from Sigma-Aldrich (Milwaukee, WI), and lithium aluminum hexahydride (Li3AlH6) from GFS Chemicals (Powell, OH), were used as-received without any further purification. All of the material handling was carried out in a glovebox filled with purified argon gas (99.999%) in the presence of an oxygen scavenger and a drying agent to protect samples and starting materials from oxidation and/or hydroxide formation (O2 < 1 ppm; H2O < 2 ppm). Approximately 2.0 g mixtures of Li3AlH6/2LiBH4 and of 0.5Al/LiBH4 were prepared in the glovebox, and 4 wt % of TiCl3 was added as a catalyst in each mixture to enhance the hydrogenation and dehydrogenation kinetics. It is known that the temperature during high-energy ball milling can rise above 200 °C,42,43 and this condition would cause the decomposition of Li3AlH6. Thus, low-energy ball milling was done in a jar-roll mill under an argon atmosphere to avoid temperature buildup, minimize the loss of H2, and achieve a uniform mixture. The ratio

Table 1. Designation of Samples

Li3AlH6/2LiBH4/4

after dehydro-

after rehydro-

after milling

genation

genation

S1

S2

S3

S4

S5

S6

wt %TiCl3 0.5Al/LiBH4/4 wt %TiCl3

of ball to powder in the mill was 35:1 by weight, and the milling time was 24 h. The milling jar, with an inner volume of 160 mL, was sealed by a Viton-type O-ring, which kept the inside of atmosphere inert during milling. The hydrogen release properties of the mixtures were examined by the use of a thermogravimetric analyzer (TGA, Shimadzu TGA50) in which 5 mg samples were heated under flowing argon up to 380-450 °C at a heating rate of 2 °C/min and then held at the final temperature for a predetermined length of time. This equipment was specially designed and built to be used inside of the argon-filled glovebox equipped with a gas purification system, which permitted the TGA analysis without exposing the sample to air. The hydrogenation property of the dehydrogenated product was evaluated by the use of a custom-made autoclave with a hydrogen pressure limit of 35 MPa and a maximum operating temperature of 500 °C. The reactor was charged in the glovebox with 300 mg of the mixtures and sealed. It was then attached to a gas-vacuum manifold and quickly evacuated to remove most of the argon remaining therein. In the rehydrogenation process, the mixtures were held at 380-450 °C for 4-6 h under vacuum to ensure complete release of hydrogen and then kept for 12-18 h under 15-24.1 MPa of hydrogen pressure at a predetermined temperature. All of the hydrogenation and dehydrogenation experiments were repeated twice, and the results were always close, within (0.05 wt %. The identification of the phases in the reactants and products before and after the TGA was carried out using an X-ray diffractometer (XRD, Siemens D5000) with Ni-filtered Cu KR radiation (λ = 1.5406 Å). Each sample was mounted in the glovebox on a glass slide and covered with a Kapton tape as a protective film. The X-ray intensity was measured over diffraction 2θ from 10 to 100° with a scanning rate of 0.02°/s. From the XRD peak broadening, the crystallite size of the sample were obtained by applying Stokes and Wilson’s formula.44 A scanning electron microscope (SEM, TOPCON SM-300) was employed to observe the microstructure of the samples before and after milling and determine the particle size. The samples were protected by a conductive tape from exposure to air during the transfer to the SEM sample chamber.

3. RESULTS AND DISCUSSION 3.1. Reactions in the Li3AlH6 þ 2LiBH4 System. In our previous work on the Li—Al—B—H system,29 a milling time of 24 h in a jar-roll mill (referred to as low-energy milling hereafter) gave degrees of mixing and particle size reduction that did not change much by a further increase in milling time. Thus, the Li3AlH6/2LiBH4/4 wt %TiCl3 mixture was subjected to 24 h of low-energy milling in an argon-filled canister. It is noted that the mixtures after milling, dehydrogenation, and rehydrogenation are referred to as S1, S2, and S3, respectively, hereafter in this paper and summarized in Table 1. 6041

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Figure 1. SEM micrographs of Li3AlH6/2LiBH4/4 wt %TiCl3 (A) before milling and (B) after 24 h of milling.

Figure 2. XRD patterns of Li3AlH6/2LiBH4/4 wt %TiCl3 (A) before milling and (B) after milling. (Part of the broad peak on the far left is from a Kapton tape used to cover the powders.)

SEM was used to examine the micrographs of a TiCl3-doped Li3AlH6/2LiBH4 mixture (A) before milling and (B) after milling, as shown in Figure 1. The milled particles are irregular in shape and are mostly agglomerated (making the particle size appear larger), while the primary particle size has been reduced to less than 1 μm, as shown in Figure 1B, from the coarse particle size of as-received powder of less than 5 μm, as shown in Figure 1A. The milled powder was subsequently analyzed by XRD and TGA to study their dehydrogenation and rehydrogenation characteristics. It has been reported that the formation of the nanocrystalline structure produced by ball milling results in a significant improvement of hydrogen release and uptake kinetics.45-47 However, high-energy ball milling or longer milling time may cause unexpected reactions and changes in the original phases. XRD analyses were conducted to determine the effect of the milling method used in this study on the particle size, lattice defects, and phases. In Figure 2, which shows the XRD patterns of the mixture before and after milling, the peaks marked with 1, 2, and 3 are attributed to Li3AlH6, LiBH4, and TiCl3 phases, respectively. It is seen that there is no trace of Fe, which may come from the erosion of the milling tools or the formation of any new phases. In other words, the LiBH4, Li3AlH6, and TiCl3 components are

Figure 3. TGA result for the milled Li3AlH6/2LiBH4/4 wt %TiCl3 obtained at a heating rate of 2 °C/min up to 450 °C followed by isothermal hold. It is noted that the vertical line segment at the end of the curve represents weight loss during the isothermal hold at the final temperature of 450 °C.

preserved under the milling conditions of this work. In addition, despite the presence of large particles, the milled TiCl3-doped Li3AlH6/2LiBH4 mixture presents broad diffraction peaks and lower intensities in the XRD pattern, as shown in Figure 2B, compared with those of the mixture before milling, shown in Figure 2A. The broadening of diffraction peaks with lower intensities indicates the refinement of the crystallite size and the presence of the lattice microstrain in the milled TiCl3-doped Li3AlH6/2LiBH4 mixture. The average crystallite sizes calculated by applying Stokes and Wilson’s formula44 to the strongest peak for the mixtures before and after milling are 1.4 μm and 24 nm, respectively. Thus, the milling method used in this work did not cause any undesirable reactions, which sometimes occur during the milling of materials with low dehydrogenation temperatures. Figure 3 shows the TGA curve of the milled mixture of TiCl3doped Li3AlH6/2LiBH4 (S1). The TGA experiment was run in argon, in which the sample was heated at a heating rate of 2 °C/min up to 450 °C and then held at this temperature for continued decomposition. It can be seen that the total weight loss amounted to 8.8 wt % of the initial weight, which takes place 6042



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Figure 4. XRD profiles of Li3AlH6/2LiBH4/4 wt %TiCl3 (A) after milling, (B) after dehydrogenation at 220 °C for 20 min, and (C) after dehydrogenation at 450 °C for 12 h and (D) the rehydrogenated product of the dehydrogenated mixture at 450 °C under 24.1 MPa of hydrogen pressure for 12 h. (Part of the broad peak on the far left is from a Kapton tape used to cover the powders.)

within the temperature range from 81 to 450 °C. The dehydrogenation process consists of two steps, which are identified by the changes in the rate of the weight loss. These steps can be described by the following reactions Li3 AlH6 f 3LiH þ Al þ 1:5H2

ð3Þ

2LiBH4 þ Al f 2LiH þ AlB2 þ 3H2

ð4Þ

resulting in the overall reaction given by Li3 AlH6 þ 2LiBH4 f 5LiH þ AlB2 þ 4:5H2

ð5Þ

The first step between 81 and 226 °C corresponds to reaction 3, which causes the loss of 3.1 wt % of the initial weight (including those of LiBH4 and TiCl3), assuming that all of the weight loss is due to the release of hydrogen. It has been reported that TiCl3catalyzed Li3AlH6 has about a 30 °C lower dehydrogenation temperature, while Li3AlH6 starts decomposing at around 210 °C without a catalyst.13 The second step between 226 and 450 °C, which corresponds to reaction 4, releases another 6.1 wt % hydrogen, making the overall theoretical capacity 9.2 wt % for the mixture, according to reaction 5. It is noted that the vertical line segment at the end of the curve represents the weight loss during the isothermal hold at the final temperature of 450 °C. It is known that modification of LiBH4 with additives such as metal oxides and chlorides also lowers the hydrogen release temperature from 400 to 200 °C.16,19,21,23,24,26,28,30,33 The dehydrogenation steps listed above were confirmed by X-ray diffraction analyses on the raw materials and the products. Figure 4A-C compares the XRD profiles of the milled mixture of TiCl3-doped Li3AlH6/2LiBH4 before and after dehydrogenation by heating up to 220 or 450 °C and holding for a certain time. Crystalline phases were identified by comparing the experimental data with the JCPDS files from the International Center for Diffraction Data. Pattern A in Figure 4 for the milled sample before dehydrogenation (S1) shows the reactants Li3AlH6, LiBH4, and TiCl3. The intermediate product after partial dehydrogenation at a specific temperature of 220 °C for

Figure 5. TGA curves for (A) the rehydrogenation product of dehydrogenated Li3AlH6/2LiBH4/4 wt %TiCl3 obtained by rehydrogenating at 450 °C under 24.1 MPa of hydrogen pressure compared with (B) asmilled Li3AlH6/2LiBH4/4 wt %TiCl3. The curves show hydrogen release under argon at a heating rate of 2 °C/min up to 450 °C followed by isothermal hold. It is noted that the vertical line segment at the end of the curve represents weight loss during the isothermal hold at the final temperature of 450 °C.

20 min, presented as pattern B in Figure 4, is composed of Al or Al þ LiH (Al and LiH peaks largely overlap), LiBH4, and TiCl3. This confirms that the first dehydrogenation step corresponds to reaction 3, and LiBH4 is still present in the sample. Pattern C shows the XRD result for the sample after dehydrogenation at 450 °C for 6 h, clearly indicating that Li3AlH6 and LiBH4 are absent in the sample by being consumed by the reaction. In this pattern, all of the peaks can be indexed to be those of LiH or AlB2, which indicates that reaction 4 is complete. Thus, it can be concluded that the overall dehydrogenation path follows reactions 3 and 5. In order to evaluate the reversibility of the Li3AlH6/2LiBH4/ 4 wt %TiCl3 system, S2, the dehydrogenated products of reaction 5, was hydrogenated in a custom-made autoclave under 24.1 MPa of hydrogen pressure and 450 °C for 12 h. The XRD profile of the rehydrogenated products (S3) is shown in Figure 4D. It is seen that the peaks of AlB2 and some of LiH are absent, which indicates that they are used by the hydrogenation reaction. Small peaks of LiBH4 are also seen. In this figure, the peaks marked with 4 are indexed to the LiH phase, and those marked with 5 represent Al. It is noted, however, that the XRD peaks of LiBH4 are weak after the hydrogen uptake reaction. In addition, monoclinic Li3AlH6, which is identified by the characteristic double peak at 2θ values of approximately 22°, is absent in the hydrogenated product. This indicates that the hydrogenation was incomplete under the above experimental conditions. To further confirm the rehydrogenation reaction, TGA was performed on the rehydrogenated sample in combination with XRD analysis. Figure 5A shows the TGA profile of S3. It can be seen that the sample took up hydrogen amounting to only about 3.8 wt % of the rehydrogenated products. It is again noted that the vertical line segment at the end of the curve represents the weight loss during the isothermal hold at the final temperature of 450 °C. The dehydrogenation of S3 appears to be a one-step process rather than the two-step process shown in Figure 5B for the initial mixture following reactions 3 and 4. Although the partial rehydrogenation requires further studies, the potential factor that may explain it is that LiBH4 still requires a 6043



Figure 6. XRD patterns of 0.5Al/LiBH4/4 wt %TiCl3 (A) after milling, (B) after dehydrogenation at 400 °C for 6 h, and (C) after dehydrogenation at 450 °C for 6 h. (Part of the broad peak on the far left is from a Kapton tape used to cover the powders.)

higher rehydrogenation temperature and pressure, even when destabilized by either element substitution or additive interaction.16,19,21,23,24,26,28,30,33 In addition, the formation of the Li3AlH6 phase is influenced by the heating rates during the hydrogenation process. It has been reported that the Li3AlH6 phase formed only when the heating rate was greater than 5 °C/min.39 The above XRD and TGA results indicate that the dehydrogenated products of Li3AlH6/2LiBH4/4 wt %TiCl3 mixtures can only be partially rehydrogenated in 12 h under 24.1 MPa of hydrogen pressure and 450 °C. However, the presence of aluminum is an important factor in causing the rehydrogenation to occur at a much lower temperature at which dehydrogenation product from LiBH4 alone does now show any rehydrogenation.16,19,21,23,24,26,28,30,33 In regard to the role as a destabilizing agent, the effects of aluminum metal on the hydrogen release/ uptake reactions of LiBH4 will be further discussed in the following section to better elucidate the reaction mechanisms including dehydrogenation pathway and reversibility. 3.2. Reactions in the 0.5Al þ LiBH4 System. It has been reported that the thermal decomposition of LiBH4 could be significantly accelerated by mechanical milling with Al in a 2:1 molar ratio,26,28,30,33 for which the theoretical hydrogen content is 8.5 wt % according to reaction 4 in section 3.1. However, this system suffers from a serious capacity loss during the dehydrogenation and rehydrogenation cycles,26,28,30,33 and the degradation mechanism is not well understood. Thus, in order to further investigate how aluminum metal affects the reaction as a destabilizing additive, a mixture of 0.5Al/LiBH4/4 wt %TiCl3 was prepared by low-energy milling for 24 h under an Ar atmosphere. The reaction products after milling, dehydrogenation, and rehydrogenation are designated as S4, S5, and S6 for the mixtures with 0.5Al, respectively, as indicated in Table 1. In order to determine if the dehydrogenation of S1 indeed follows reaction 4 under the same dehydrogenation conditions as those presented in section 3.1, XRD was used to analyze the reaction products. Specifically, about 300 mg of S4 was charged in an autoclave and sealed within a glovebox. The reaction vessel was then taken out of the glovebox and heated to 400-450 °C under vacuum with a heating rate of 5 °C/min. After holding the autoclave at a predetermined temperature for 6 h, it was transferred back into the glovebox with the reaction product still in it.

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Figure 7. TGA result obtained at a heating rate of 2 °C/min up to 380 °C for 0.5Al/LiBH4/4 wt %TiCl3 milled for 24 h. Curve (A) shows the hydrogen release under argon, and curve (B) shows the temperature profile corresponding to curve (A).

Figure 6 compares the XRD profiles of S4 and the dehydrogenated products heated at 400 and 450 °C for 6 h. In Figure 6A, representing S4, the peaks marked with 1, 2, and 3 are attributed to Al, LiBH4, and TiCl3 phases, respectively. It shows that these components are preserved without forming any new phases under the milling conditions of this work. Patterns B and C in Figure 6 show the XRD results for the dehydrogenated products of heating at 400 and 450 °C for 6 h, respectively, which clearly indicate that Al and LiBH4 are absent in the sample by being consumed by the H2 release reaction. The presence of LiAl in addition to LiH and AlB2 indicates that the dehydrogenation involves reactions other than just reaction 4. Moreover, the peak intensities of LiAl phases in Figure 6C are significantly higher than those in Figure 6B as the dehydrogenation temperature was raised from 400 to 450 °C. Recently, Friedrichs et al.33 reported that the LiAl phase was formed at 450 °C by the reaction of LiH and Al during the dehydrogenation of the 0.5Al/LiBH4 system in vacuum, although the LiH and AlB2 phases were observed even after 15 h in these conditions. The formation of LiAl from LiH and Al increases hydrogen release by the dehydrogenation of LiH, which is otherwise difficult. A local deviation from the ratio LiBH4/Al = 2:1 might cause an incomplete reaction between LiBH4 and Al at first. Then, the presence of unreacted Al at the high temperature of 450 °C and the low pressure causes the LiAl formation. Although the formed LiAl can, in principle, react with B and H2 to regenerate LiBH4 during rehydrogenation, the incomplete contact between the solid phases may make it difficult. This in turn would result in a capacity loss during the subsequent cycles. The milled mixture of 0.5Al/LiBH4/4 wt %TiCl3 (S4) was subsequently subjected to TGA to study their dehydrogenation behavior, as shown in Figure 7. The TGA experiment was run at a heating rate of 2 °C/min up to 380 °C, after which the temperature was held at this value for 6 h. The dehydrogenation process of S4, which consists of one step, began at about 200 °C and accelerated at ∼300 °C. It can be seen that the total weight loss amounted to 8.4 wt % of the initial weight, which took place within the temperature range from 200 to 380 °C. The predetermined target temperature, 380 °C, was selected for the following reasons. First, the weight loss percentages recorded by TGA (8.4 wt %) matches well with the stoichiometric values of reaction 4 (8.5 wt %). The LiAl phase formed above 400 °C, 6044



Figure 8. XRD profiles of 0.5Al/LiBH4/4 wt %TiCl3 (A) after milling, (B) after dehydrogenation at 380 °C for 4 h, and (C) after rehydrogenation at 380 °C under 15 MPa of hydrogen pressure for 18 h. (Part of the broad peak on the far left is from a Kapton tape used to cover the powders.)

shown in Figure 6 and also reported by Friedrichs et al.,33 might be one of the possible reasons for capacity degradation during the subsequent H2 release/uptake cycles. Therefore, a higher temperature above 380 °C was not considered in this study. Second, the sample dehydrogenated below 380 °C contains a considerable amount of Al, based on the XRD and TGA results (not shown), which is caused by an incomplete reaction between LiBH4 and Al at this temperature. In order to further verify whether the dehydrogenation reaction follows reaction 4, X-ray diffraction was conducted on the sample before and after hydrogenation in conjunction with TGA. Figure 8A and B compares the XRD profiles of S4 and S5, respectively, which are the reaction products after milling and dehydrogenation by heating up to 380 °C and holding for 4 h. The XRD result for S4 in Figure 8A shows the phases of the reactants Al, LiBH4, and TiCl3. On the other hand, Figure 8B, which is the XRD pattern after dehydrogenation, shows only the peaks attributed to LiH, AlB2, and TiCl3 phases. It is clearly seen that S5 is a mixture of LiH and AlB2 and that there is no formation of the LiAl phase when heated only up to 380 °C. This suggests that the hydrogen release reaction at 380 °C followed reaction 4. To test the reversibility of dehydrogenated products of reaction 4, S5 was hydrogenated by using an autoclave under 15 MPa of hydrogen pressure at 380 °C for 18 h. The hydrogenated products (S6) were analyzed by XRD and TGA. The XRD profile of S6 is shown in Figure 8C, which suggests that the dominant phases consist mainly of LiBH4 and Al, and thus the significant rehydrogenation was achieved at the above experimental conditions. Figure 9 presents the TGA curve of hydrogenated products (S6) with the same temperature profile mentioned above, revealing that it has gained about 5.8 wt % hydrogen from the treatment. It is noted that the hydrogen content in S6 is less than that of the milled mixture (S4). In fact, numerous articles published recently on similar material systems involving Al and LiBH4 showed that a capacity loss during H2 release and uptake cycles takes place, and the sample degenerates successively with respect to its hydrogen capacity.26,28,30,33 The above experimental results may be explained by the following reasoning. The reduced reaction enthalpy change of the LiBH4 dehydrogenation and the local deviation from the stoichiometric ratio LiH/

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Figure 9. TGA curves for (A) the rehydrogenation product of dehydrogenated 0.5Al/LiBH4/4 wt %TiCl3 obtained by rehydrogenating at 380 °C under 15 MPa of hydrogen pressure for 18 h compared with (B) as-milled 0.5Al/LiBH4/4 wt %TiCl3. Curves (A) and (B) show the hydrogen release under argon with a heating rate of 2 °C/min up to 380 °C. Curve (C) shows the temperature profile corresponding to curves (A) and (B).

AlB2 = 2:1 cause an incomplete LiBH4 formation reaction during rehydrogenation. The considerable decrease in the enthalpy of LiBH4 dehydrogenation from 69.2 down to 18.8 kJ/mol H2 in the presence of Al, as reported in the literature,26,28 places the latter enthalpy value outside of the optimum range of 3060 kJ/mol H2, implying difficulty in the rehydrogenation. In addition, it is possible that the reaction products of AlB2 and LiH are not in close contact because they are not milled after dehydrogenation. Therefore, these incomplete reactions are believed to cause the low extent of rehydrogenation, leaving a substantial amount of residual AlB2, as shown in Figure 8C. Similar to the LiBH4 systems destabilized by MgH2, TiO2, or TiCl3,16,18,19,21 the improved dehydrogenation performances of the LiBH4/Al system should also be attributed to the formation of AlB2. The above discussion indicates that only a portion of AlB2 was involved in the formation of LiBH4. On the other hand, it is also clear that the rehydrogenation reaction proceeded relatively efficiently at such a low temperature, and we believe this is promoted by the presence of Al as a catalyst. Although the results of this study demonstrate by TGA and XRD the capacity and reversibility of using LiBH4/Al as a hydrogen storage material, the full potential of the Li—Al— B—H system cannot be determined until the reaction products are fully analyzed with conclusive determination of the reaction pathway. Further investigation on the reaction paths of the Li— Al—B—H system by the use of solid-state NMR analyses of the reaction samples is presented in the accompanying Part 2.41

4. CONCLUSIONS In the present work, a mixture of LiBH4 with Li3AlH6 or Al was milled by low-energy milling, and the dehydrogenation and rehydrogenation properties of these materials were investigated by the use of SEM, XRD, and TGA. The TGA results indicated that the Li3AlH6/2LiBH4/4 wt %TiCl3 and 0.5Al/LiBH4/4 wt % TiCl3 released ∼8.8 and ∼8.4 wt % H2 when heated to 450 and 380 °C, and ∼3.8 and ∼5.8 wt % H2, respectively, were taken up during rehydrogenation. XRD results indicated that the dehydrogenated products are composed of LiH and AlB2, resulting in a remarkable improvement of the reversible storage properties of LiBH4 in terms of the temperature and pressure for H2 release/ uptake. Although the rehydrogenation reaction did not fully 6045


The Journal of Physical Chemistry C proceed in the limited temperature and pressure ranges tested, the presence of Al was determined to be an important factor in lowering the rehydrogenation temperature to a level at which the dehydrogenation product from LiBH4 alone does not show any degree of rehydrogenation.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (801) 581-5491. Fax: (801) 581-4937. E-mail: h.y.sohn@ utah.edu. Present Addresses †

Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352. ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL 60439.

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