Appl. Phys. A 89, 963–966 (2007)
Applied Physics A
DOI: 10.1007/s00339-007-4198-z
Materials Science & Processing
xiang-dong kang ping wangu lai-peng ma hui-ming cheng
Reversible hydrogen storage in LiBH4 destabilized by milling with Al Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China
Received: 7 June 2007/Accepted: 13 June 2007 Published online: 25 July 2007 • © Springer-Verlag 2007
LiBH4 possesses a high hydrogen content, and though it is highly stable, its restoration from LiH + B + H2 can only be accomplished under unacceptable high temperature and pressure conditions (650 ◦ C and 15 MPa). Recently, it has been reported that destabilizing LiBH4 by, i.e., MgH2 , transition metal oxides and chlorides presents a promising approach to exert its potential for hydrogen storage. In the present study, we find that simple mechanical milling with Al in a mole ratio of 2 : 1, markedly improves the reversible dehydrogenation performance of LiBH4 . The system possesses a theoretical capacity of 8.5 wt. % and could be reversibly operated at 400–450 ◦ C. The combined property and phase examinations suggest that the observed property improvement should be associated with the formation of AlB2 in the dehydriding process. Further cyclic examination found that the system suffered from a serious capacity loss in the dehydriding/rehydriding cycles. A better understanding of the degradation mechanism may provide a means for further material property improvement.
100 K over its melting point [2]. LiBH4 → LiH + B + 3/2H2 .
(1)
ABSTRACT
PACS 81.05.Zx;
1
84.60.Ve; 82.30.-b; 82.33.Pt; 82.65.+r
Introduction
The development of a safe and efficient onboard hydrogen storage system has generally been recognized as a key technical challenge in promoting the hydrogen fuel cell-powered vehicle (HFCV). The commercialization of the HFCV requires that the hydrogen storage system must meet the stringent combined requirements for H-capacity, kinetics, cycle life, safety, and cost etc. Decades of extensive research on metal/alloy hydrides and nanostructured carbon materials have been largely frustrated by their low H-capacity. Recently, light metal complex hydrides have become an active research frontier [1]. Among complex hydrides, LiBH4 is of particular interest due to its extremely high gravimetric and volumetric hydrogen densities (18.5 wt. % and 121 kg H2/m3 , respectively), far exceeding the 2007 target for HFCV application set by the U.S. Department of Energy (DOE) (6.5 wt. % and 65 kg H2 /m3 ) [2]. However, the practical hydrogen storage properties of LiBH4 are poor. The hydride is so stable that an appreciable H-release following (1) only proceeds at over 650 K, about u Fax: +86 24 2397 1215, E-mail:
[email protected]
On the other hand, the temperature and pressure conditions for restoring the hydride via the reverse reaction of (1) are too harsh, far beyond the acceptable level for practical operation. Therefore, novel strategies and methods have to be developed to explore the availability of LiBH4 for hydrogen storage application. Züttel et al. [2] claimed that by mixing with SiO2 , the onset decomposition temperature of LiBH4 could be lowered by about 200 ◦ C, to around 200 ◦ C. Quite recently, Au et al. [3] used the similar strategy but achieved a more pronounced property improvement. It was found that mechanically milling LiBH4 with selected metal oxides or metal chlorides produced not only a destabilized hydride, but also a reversible hydrogen storage system. With the aid of the additive, the decomposed LiBH4 could be recharged at 600 ◦ C under 7 MPa hydrogen. Based on coupled experimental and theoretical studies, Orimo et al. [4–6] proposed an alternative approach to destabilize LiBH4 . They reported that the decomposition temperature of LiBH4 was lowered by about 30 ◦ C by partial cation substitution of Li by Mg, which has a larger electronegativity [4, 5]. The hydrogen evolution processes of most of the above-mentioned systems utilize a similar general procedure, that of thermal decomposition of LiBH4 destabilized by additives or their derivatives. Vajo et al. [7] revealed another approach to utilize the huge amount of hydrogen in LiBH4 . They mechanically milled LiBH4 + 1/2MgH2 together with a small amount of TiCl3 catalyst, thus producing a system that can reversibly store 8 − 10 wt. % hydrogen. The adjustment of the reaction pathway resulted in a decrease of 25 kJ/mol of H2 in the hydrogenation/dehydrogenation enthalpy compared with that of pure LiBH4 . The aim of the present study is to explore the function of Al to destabilize LiBH4 for reversible hydrogen storage. It was found that by mechanically milling LiBH4 with Al powder and a small amount of TiF3 catalyst, a reversible hydrogen storage system was produced. Preliminary structural characterization was also performed to understand the de-/rehydriding processes and the results are shown. 2
Experimental
The starting materials, LiBH4 (95%, gas-volumetric), Al powder (99.97%, 325 mesh), and TiF3 powder,
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were all purchased from Sigma-Aldrich Corp. and used as received. The mixtures of LiBH4 + 1/2Al + 0.04 TiF3 and LiBH4 + 0.04 TiF3 were mechanically milled for 10 h under Ar atmosphere by using a Fritsch 7 planetary mill at 400 rpm in a stainless steel vial together with eight steel balls (10 mm in diameter). The ball-to-powder ratio was around 40 : 1. All sample operations were performed in an Ar-filled glovebox equipped with a recirculation system to keep the H2 O and O2 levels below 0.1 ppm. Dehydriding/rehydriding behaviors of the samples were examined by using a carefully calibrated Sievert’s type apparatus. In a typical cyclic experiment, the sample was held at 450 ◦ C for dehydrogenation and at 400 ◦ C for rehydrogenation with an initial pressure of < 100 Pa and 10 MPa, respectively. To minimize H2 O/O2 contamination, the highpurity hydrogen gas (99.999%) was further purified using a hydrogen storage alloy system. Hydrogen storage capacity was determined with respect to the total weight of the sample, including the catalyst. The samples were characterized by powder X-ray diffraction (XRD) (Rigaku D/MAX-2500, Cu K α radiation) and a scanning electron microscope (SEM) (JSM 6301F) equipped with an energy dispersive X-ray (EDX) analysis unit (Oxford). All the sample preparations were operated in the Ar-filled glovebox. To minimize the H2 O/O2 contamination during the XRD measurement, a small amount of grease was used to cover the surface of the samples. The SEM samples were prepared by spreading the dry powder on conductive carbon double face tape supported by a copper pole. The subsequent sample transfer into the SEM equipment was performed in a specially designed Ar-filled device. 3
Results and discussion
The thermal decomposition process of LiBH4 could be significantly accelerated by mechanical milling with Al in a 2 : 1 molar ratio. Figure 1a presents a typical comparison of the dehydriding profiles between the samples LiBH4 + 1/2Al + 0.04 TiF3 and LiBH4 + 0.04 TiF3 that were mechanically milled for 10 h under Ar atmosphere. Here, TiF3 was used in both samples as a catalyst. The sample with Al additive was observed to release over 7.2 wt. % hydrogen in about 3 h at 450 ◦ C. This value corresponds to 90% of the theoretical hydrogen content of the reaction following (2), and was confirmed by XRD examination as stated below. LiBH4 + 1/2 Al ↔ LiH + 1/2AlB2 + 3/2H2 .
Comparison of the dehydriding (DH)/rehydriding (RH) profiles between (a) LiBH + 1/2Al + 0.04 mol TiF3 and (b) LiBH4 + 0.04 mol TiF3 . Both samples were prepared by mechanical milling under Ar atmosphere for 10 h
FIGURE 1
period. For both samples, the observed pressure increase during the initial period is due to the temperature increase as hydrogen gains access to the reactor. The dehydring/rehydriding reversibility as well as the proposed reaction processes of the LiBH4 -Al-TiF3 system were further confirmed by XRD examination. Figure 2 presents the XRD patterns of the sample in the as-milled, dehydrogenated and rehydrogenated states. In all cases, no Ticontaining phase was detected due to the small added amount of TiF3 , as well as the nanostructure that was likely produced during the intensive milling process. In the as-milled sample, only LiBH4 and Al were detectable. Judging from the broadened diffraction peaks, both phases were nanocrystalline with an average grain size of 10 nm. As expected, AlB2 was generated in the dehydriding process. However, another solid product, LiH, was barely identified due to its fine
(2)
In contrast, the sample LiBH4 + 0.04 TiF3 released only 5.5 wt. % hydrogen under identical measurement conditions. This value was far less than the theoretical value of 11.5 wt. % according to (1). Of particular interest, it was found that the addition of Al rendered reversible rehydrogenation in the the system at relatively moderate conditions. During exposure to 10 MPa hydrogen at 400 ◦ C, as seen from Fig. 1b, the dehydrogenated sample with Al additive could reabsorb considerable amounts of hydrogen within approximately 100 min. In the case of the LiBH4 + 0.04 TiF3 sample, however, no appreciable hydrogen absorption was observed within the examined time
XRD profiles of LiBH4 + 1/2Al + 0.04 mol TiF3 mixture milled under Ar atmosphere for 10 h: (a) as-milled; (b) in the dehydrogenated state in the first cycle; (c) in the hydrogenated state in the second cycle. The insert gives the magnified profiles of the marked region and the corresponding multi-peaks lines fitted by Lorentz function FIGURE 2
KANG et al.
FIGURE 3
Reversible hydrogen storage in LiBH4 destabilized by milling with Al
The dehydriding curves of the material in the first 4 cycles
grain size and the closeness of its peak position to that of AlB2 and Al. However, in a careful examination of the peak around 2θ = 44.8◦ (as given in the inset in Fig. 2), the LiH (2θ = 44.5◦ ) peak could be identified. This, together with the small peak around 2θ = 64.7◦ , evidenced the formation of LiH. Here, it should be noted that a considerable amount of Al was also detected in the dehydrogenated state, which will be discussed below. The dehydrogenated sample still contained considerable a amount of Al, which was expected to be completely consumed in the dehydriding process following (2). This may, at least partially, explain the capacity gap between the theoretical value and that which was practically obtained. After recharging the sample at 400 ◦ C, the peaks of AlB2 and LiH disappeared and LiBH4 was identified, indicating the complete reversibility of the system. Similar to the LiBH4 systems destabilized by MgH2 , TiO2 or TiCl3 etc., in which the generated MgB2 or TiB2 stabilized the dehydrogenated state [2–5, 7], the improved dehydrogenation performances of the LiBH4 /Al system should also be associated with the formation of AlB2 . This is understandable from a thermodynamic perspective. Due to the large formation enthalpy of AlB2 (−151 kJ/mol), the enthalpy change of the dehydriding reaction has been decreased from 69.2 for (1) to 18.8 kJ/mol H2 for (2) [2, 8]. The markedly reduced reaction enthalpy means a significant increase of dehydriding plateau pressure, thus promising an improved dehydriding performance. On the other hand, however, the low enthalpy change falls outside the optimum range of 30 ≤ ∆H ≤ 60 kJ/mol H2 , thus presenting a challenge in recharging the hydride. In practice, the problematic thermodynamics appear to be compensated by the kinetic enhancement since, as observed, the Al destabilized LiBH4 system could be completely recharged while the thermodynamically more favorable reaction LiH + Al + 3/2H2 → LiAlH4 (∆HR = − 28.5 kJ/mol) did not proceed under identical conditions [9]. This clearly suggests that the energy barrier of the rehydrogenation reaction of LiBH4 was significantly lowered through the change of the reaction pathway. In this regard, further theoretical studies are required to understand the mechanism. In further examination of the cyclic stability, it was found that the LiBH4 /Al system underwent serious capacity degra-
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dation when the cycle number increased. As seen in Fig. 3, the H-capacity decreased from the initial 7.2 to about 3 wt. % after four cycles, with a pronounced decrease of 2.1 wt. % over the first two cycles. Two possibilities are proposed that might be responsible for the capacity loss. The first is the loss of boron. In the study of the LiBH4 system destabilized by transition metal oxides or chlorides, Au et al. [3] found that a trace amount of BH3 was released during the decomposing process of LiBH4 . Such a loss of boron directly resulted in a gradual decrease of the amount of H-source LiBH4 . In our effort to check this point, however, we encountered difficulty in thermal analyses measurements. It was found that heating the sample to over 400 ◦ C resulted in considerable volatilization of Li- or B-containing species and consequently exposed the Pt holder to a no revert contamination. Currently, we are trying to solve this problem by re-designing the crucible. The second possibility is that the capacity loss is due to an incomplete reaction between LiBH4 and Al. In this regard, the XRD results shown in Fig. 2 may provide an important hint. After ball milling and the first dehydriding reaction, some Al was still detected, indicating the local deviation of the stoichiometric ratio of LiBH4 : Al = 2 : 1. In the following rehydriding/dehydriding reactions, the repeated reconstruction and decomposition of LiBH4 as well as the atomic diffusion at a relatively high operation temperature may possibly result in an aggravated phase segregation of LiBH4 /Al and/or LiH/AlB2 . As demonstrated, the addition of Al has resulted in a pronounced improvement of the reversible dehydrogenation of LiBH4 . However, substantial property breakthrough is still required before the LiBH4 /Al system can be subjected to any practical consideration. Therefore, an understanding of the mechanism of the problematic capacity loss indicates the direction of our future efforts. A novel catalyst needs to be developed to facilitate the reaction between LiBH4 and Al, suppressing the formation of boron hydride(s). Advanced preparation technology is also required to obtain a more homogeneous mixing and decreased particle size of the materials. 4
Conclusion
The potential of pure LiBH4 for hydrogen storage application is greatly hindered by the harsh processing temperature and pressure conditions necessary. Here, we found that the reversible dehydriding performances of LiBH4 could be markedly improved by simple mechanical milling with Al in a mole ratio of 2 : 1. The Al destabilized system can be operated at 400 – 450 ◦ C and largely completes the dehydriding/rehydriding reactions within several hours. According to the XRD results, the property improvement should be associated with the formation of AlB2 phases in the dehydriding process. While possessing a theoretical hydrogen capacity of 8.5 wt. %, the Al destabilized LiBH4 system was observed to suffer from a serious cycling degradation. Currently, it is believed that the loss of boron via vaporization of boron hydride(s) and the incomplete reaction between LiBH4 and Al are responsible for the capacity loss. Further property improvement is therefore expected from the development of novel catalyst and advanced preparation methods.
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ACKNOWLEDGEMENTS The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences and the National Natural Science Foundation of China (Project 50571099) are gratefully acknowledged.
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