ARTICLE pubs.acs.org/JPCC
Improved Hydrogen Storage Properties of LiBH4 Destabilized by in Situ Formation of MgH2 and LaH3 Yifan Zhou, Yongfeng Liu,* Wei Wu, Yu Zhang, Mingxia Gao, and Hongge Pan State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
bS Supporting Information ABSTRACT: A reactive composite of LiBH4xLa2Mg17 was successfully prepared by means of mechanochemical reaction under 40 bar of H2. It was found that MgH2 and LaH3 were readily formed in situ during high-pressure ball milling, and a strong dependency of hydrogen storage performance of the LiBH4xLa2Mg17 composites on the content of La2Mg17 was observed. The as-prepared LiBH40.083La2Mg17 composite under 40 bar of H2 exhibits superior hydrogen storage properties as ∼6.8 wt % of hydrogen can be reversibly desorbed and absorbed below 400 °C. It was also purposed that the selfdecomposition of MgH2 first occurred to convert into Mg with hydrogen release upon dehydrogenation and subsequently catalyzed the reaction of LiBH4 and LaH3 to liberate additional hydrogen along with the formation of LaB6 and LiH. The in situ formed MgH2 and LaH3 provide a synergetic thermodynamic and kinetic destabilization on the de/hydrogenation of LiBH4, which is responsible for the distinct reduction in the operating temperatures of the as-prepared LiBH4xLa2Mg17 composites.
1. INTRODUCTION One of the key challenges for using hydrogen as an energy carrier is how to store it in safe, efficient, and reversible manner.1 Hydrogen storage materials that chemically or physically bind the hydrogen have stimulated intense interest regarding safety and energy density.2 Particular attention has been focused on lightmetal complex hydrides due to their high gravimetric and volumetric hydrogen capacity.310 Among them, lithium borohydride, LiBH4, has been widely accepted as a potential candidate as it offers a gravimetric hydrogen density of 18.5 wt % and a volumetric hydrogen density of 121 kg/m3.11 However, the release of hydrogen from LiBH4 proceeds at a high temperature of over 400 °C, and the rehydrogenation requires even harsher conditions at 600 °C and 150 bar of hydrogen pressure, which limits its practical applications for on-board hydrogen storage.12 In order to be a viable hydrogen storage material, LiBH4 must be fully reversible and capable of operating under reasonable conditions (low temperature and moderate pressure). Considerable effort has been devoted to reducing the desorption enthalpy as well as enhancing hydrogenation/dehydrogenation kinetics via inducing various additives including metals, halides, oxides, amides, and hydrides.1123 It was found that mixing 25 wt % LiBH4 with the SiO2 of 75 wt % successfully reduced the starting dehydrogenation temperature from 400 to 200 °C.11 Xu et al. demonstrated that all of the hydrogen was released from LiBH4 catalyzed by carbon-supported Pd nanoparticles with a doping level of 50 wt % below 580 °C.13 The effect of transition metals (Ti, V, Cr, Ni, etc.),1416 metal halides (TiCl3, TiF3, ZnF2, AlF3, ZnCl2, etc.),1720 and metal oxides (TiO2, V2O5, ZrO2, SnO2, etc.)21,22 on hydrogen r 2011 American Chemical Society
desorption behaviors of LiBH4 has also been widely studied. Such various studies have supported that most of these additives could effectively reduce the dehydrogenation temperatures of LiBH4 and enhance the dehydrogenation kinetics, but compromise its dehydrogenation capacity significantly in the mean time.1423 Recently, thermodynamic destabilization was successfully achieved by combining LiBH4 with reactive additives that form metal borides instead of boron upon dehydrogenation.24 Such phenomenon can decrease the overall enthalpy for dehydrogenation and increase the equilibrium hydrogen pressure, which lead to a significant hydrogen storage reversibility improvement and operating temperature reduction. As reported by Vajo et al.,25 the desorption enthalpy of the 2LiBH4MgH2 combination was lowered by 25 kJ/mol-H2 in comparison with the pristine LiBH4 due to the formation of MgB2, which results in a 250 °C reduction in the decomposition temperature. In addition, hydrogen storage in the 2LiBH2MgH2 combination was found to be completely reversible below 450 °C as described by reaction 1, which brings a step forward toward practical applications. 2LiBH4 þ MgH2 T 2LiH þ MgB2 þ 4H2
ð1Þ
Reaction 1 can theoretically release around 11.5 wt % hydrogen, only slightly lower than 13.8 wt % for the self-decomposition of LiBH4. Furthermore, thermodynamically predicted the dehydrogenation Received: October 22, 2011 Revised: December 4, 2011 Published: December 19, 2011 1588
dx.doi.org/10.1021/jp2101573 | J. Phys. Chem. C 2012, 116, 1588–1595
The Journal of Physical Chemistry C temperature for the 2LiBH2MgH2 combination is about 225 °C at 1 bar of the equilibrium hydrogen pressure. Unfortunately, high kinetic barriers make a reasonable dehydrogenation rate to be obtained at only above 400 °C. Thus, further thermodynamic reduction in desorption enthalpy and sufficient improvement in reaction kinetics at lower temperatures are eagerly needed. More recently, Vajo et al. formulated a LiBH4Mg2NiH4 combined system which displayed a concerted dehydrogenation reaction beginning at 250 °C with significantly low enthalpy (ΔH = 15.4 ( 2 kJ/mol-H2) and entropy (ΔS = 62.2 ( 3 J/K 3 mol-H2).26 The complex [NiH4]4 anion may be catalytically active due to the Ni core. The Mg(AlH4)2-catalyzed LiBH4 exhibited lower dehydrogenation temperature and faster de/ hydrogenation kinetics with respect to the individual MgH2 or Al-added LiBH4 due to the synergistic effect of MgH2 and Al which are precipitated in situ via the self-decomposition of Mg(AlH4)2.27 Similarly, Sun et al. demonstrated a synergetic effect of hydrogenated Mg3La and TiCl3 on the dehydrogenation of LiBH4.28 Approximately 3.5 wt % of hydrogen was released from the LiBH4 + HMg3La + 20 wt % TiCl3 system within 150 min at 330 °C, while the pristine LiBH4 desorbed only ca. 0.25 wt % of hydrogen under the same conditions. In the usual case, catalyst additives often reduce hydrogen storage capacity even though they could lower the dehydrogenation temperatures and improve the reversibility. In the case of the LiBH4 + HMg3La + 20 wt % TiCl3 system, the total dehydrogenation capacity was decreased to 4.3 wt % owing to the presence of 20 wt % TiCl3. Moreover, the role played by the hydrogenated MgLa alloy alone is still an enigma. In addition, it is also desirable to know how the amount of the hydrogenated MgLa alloy will affect the hydrogen storage properties of LiBH4. It is well-known that La2Mg17 can convert to MgH2 and LaH3 after taking up ∼5.3 wt % of hydrogen, which is higher than that of Mg3La (∼4.0 wt %).29,30 Additionally, hydrogen absorption/ desorption is far more rapid for La2Mg17 than for Mg.31 Hence, it is expected to achieve an optimal combination of hydrogen storage capacity and kinetic properties by introducing La2Mg17 into LiBH4 as the destabilizer. In this work, we systematically investigated the hydrogen storage behaviors of the LiBH4 xLa2Mg17 (x = 0, 0.01, 0.03, 0.05, 0.083, 0.1) combinations prepared by ball milling the corresponding chemicals under 40 bar of H2. It was found that the as-prepared LiBH40.083La2Mg17 combination exhibited superior hydrogen storage properties as it could desorb/absorb reversibly ∼6.9 wt % below 400 °C with a rather fast reaction rate. Mechanistic analyses revealed that the synergetic destabilization and catalysis effect caused by the MgH2 and LaH3 formed in situ during ball milling was the primary reason. This finding opens up the possibility for developing a LiBH4-based hydrogen storage system with high capacity, low operating temperature, and fast kinetics by optimizing the multifunctional additives which can provide both thermodynamic and kinetic destabilization.
2. EXPERIMENTAL SECTION The commercial chemicals LiBH4 (ABCR, 95%) and MgH2 (Alfa Aesar, 95%) were used as received without purification. La2Mg17 alloy was synthesized in our own laboratory by induction levitation melting in a water-cooled copper crucible under argon atmosphere. The purity of the starting metals is over 99%. The ingots were turned over and remelted twice for homogeneity.
ARTICLE
The resultant product was confirmed as single-phase La2Mg17 by means of X-ray diffraction (XRD) (Supporting Information, Figure S1). The LiBH4xLa2Mg17 (x = 0, 0.01, 0.03, 0.05, 0.083, 0.1) composites were prepared by ball milling the corresponding chemicals under 40 bar of hydrogen pressure on a planetary ball mill (QM-3SP4, Nanjing) rotating at 500 rpm for 24 h. The ballto-sample weight ratio is about 60:1. A gas valve, which can be connected to a pressure gauge for measuring the inside pressure change, was mounted on the milling jar. To ensure even mixing and milling, the mill was set to rotate for 0.2 h in one direction, pause 0.1 h, and then rotate in the reverse direction. To prevent chemicals from being contaminated by air and moisture, all handling of samples containing LiBH 4 was performed in a MBRAUN glovebox filled with pure argon (H2 O,
