international journal of hydrogen energy 37 (2012) 10750e10757 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he $ LiBH4 NH3BH3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties Junhong Luo a, Hui Wu b,c,**, Wei Zhou b,c, Xiangdong Kang a, Zhanzhao Fang a, Ping Wang a,* a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China b NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102, USA c Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, USA article info abstract Article history: Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio $ Received 17 February 2012 results in the formation of a new complex, LiBH4 NH3BH3. Its structure was successfully Received in revised form determined using combined X-ray diffraction and first-principles calculations. $ 17 March 2012 LiBH4 NH3BH3 was carefully studied in terms of its decomposition behavior and reversible Accepted 8 April 2012 dehydrogenation property, particularly in comparison with the component phases. In Available online 5 May 2012 parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding $ Keywords: structure changes in the reaction process. Our study found that LiBH4 NH3BH3 first $ Hydrogen storage disproportionates into (LiBH4)2 NH3BH3 and NH3BH3, and the resulting mixture exhibits Ammonia borane a three-step decomposition behavior upon heating to 450 C, totally yielding w15.7 wt% Lithium borohydride hydrogen. Interestingly, it was found that h-BN was formed at such a moderate tempera- $ Dehydrogenation ture. And owing to the in situ formation of h-BN, LiBH4 NH3BH3 exhibits significantly BN improved reversible dehydrogenation properties in comparison with the LiBH4 phase. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction hydrogen content (19.6 wt%), moderate thermal stability, and satisfactory air-stability. But on the other hand, the H2 release The development of hydrogen-fueled vehicles and portable from AB is severely kinetically restricted and is entangled by electronics requires advanced hydrogen storage materials the concurrent evolution of diverse volatile byproducts. In the that can store and deliver large amounts of hydrogen at past decade, several strategies have been employed to moderate temperature with fast kinetics. Recently, ammonia improve the dehydrogenation properties of AB. Addition of e borane (NH3BH3, AB for short) has received extensive atten- selected metal catalysts [4 10] or acids [11,12] enables rapid e tion as a promising hydrogen storage material [1 3]. This is H2 release from the solvolysis reactions of AB at ambient a result of its many appealing attributes, such as high temperatures. Confinement of AB in nanoscaffolds [13e15] or * Corresponding author. Tel.: þ1 86 24 2397 1622; fax: þ1 86 24 2389 1320. ** Corresponding author. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899- 6102, USA. Tel.: þ86 301 975 2387; fax: þ86 301 921 9847. E-mail addresses: [email protected] (H. Wu), [email protected] (P. Wang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.049 international journal of hydrogen energy 37 (2012) 10750e10757 10751 addition of chemical promoters [16e21] has proved to be started from the moment when the sample chamber was effective means for promoting solid-state thermolysis of AB. pushed into the preheated furnace. The solid residuals at In particular, reacting AB with alkali or alkaline-earth metal different decomposition stages were collected after heating hydrides (with an exception of MgH2) has been established as the sample to preset temperatures and then fan-cooling to a general approach for preparation of mono- or mixed-metal room temperature. Rehydrogenation of the sample was con- amidoboranes, which exhibit favorable combination of high ducted at 400 C under an initial hydrogen pressure of 10 MPa hydrogen capacity, fast hydrogen release rate and suppressed for 12 h. e volatile byproducts [22 31]. This progress stimulated further The post-milled xLiBH4/AB and the relevant neat AB and investigations on the chemical modification of AB using LiBH4 samples at varied states were characterized by XRD À À a complex anionic hydrides containing [NH2 ] or [BH4 ]. (Rigaku D/max 2500, Cu K radiation), Fourier transformation À1 Lithium borohydride (LiBH4) is a salt-like ionic crystal infrared (FTIR) spectroscopy (Bruker TENSOR 27, 4 cm containing a high density of hydrogen (18.4 wt%). Due to the resolution). Special measures were taken to minimize H2O/O2 thermodynamic and kinetic limitations that are imposed by contamination during the sample transfer process. In the the strong chemical bonds, LiBH4 exhibits high temperature routine XRD analysis, the powder samples were protected by > threshold ( 400 C) for H2 release and poor reversibility even a polymeric tape to minimize the air- and moisture- under rigorous temperature and pressure conditions [32,33].A contamination. In the study of crystal structure, the sample recent study by Wu et al. found that two H-rich materials, AB was sealed inside a 1 mm quartz glass capillary and analyzed q and LiBH4, can readily combine together to form a new layered in a continuous scan mode for over 12 h in the 2 range of $ e borohydride ammonia borane complex (LiBH4)2 NH3BH3 5 70 with a step size of 0.02 . FTIR spectra of the samples $ ((LiBH)2 AB for short) [34]. In comparison with the component were collected using the KBr-pellets method, and the obtained $ phases (LiBH)2 AB exhibits small but significant improvement spectra were normalized using OPUS 6.5 software. on the dehydrogenation properties. This material discovery, First-principles calculations based on density-functional together with the recent findings of metal amidoborane theory (DFT) were performed by using the PWSCF package ammoniates [35e39], offers new opportunities for developing [41]. We used a Vanderbilt-type ultrasoft potential with a practical source for generating molecular hydrogen. In this Perdew-Burke-Ernzerhof exchange correlation. A cutoff paper, we report the synthesis, structure, and hydrogen energy of 544 eV was found to be enough for the total energy to storage properties of a new lithium borohydride ammonia converge within 0.5 meV/atom. Car-Parrinello molecular $ $ borane compound, LiBH4 NH3BH3 (LiBH AB for short). In dynamics simulations [42] were used to help in searching for $ comparison with its analogue (LiBH)2 AB, the newly devel- the most likely crystal structures. The conventional unit cells oped LiBH$AB adopts a completely different crystal structure were used, with the cell dimensions fixed at the experimental and can deliver much more H2 via a three-step decomposition values. The initial system temperature was set to 600 K. The reaction. system was first allowed to evolve and equilibrate for 20 ps, and then the system temperature was slowly decreased to 0 K in a period of 20 ps. Structure optimizations on the resulting 2. Experimental candidate structures at 0 K were further performed with respect to atomic positions. This information was used in AB (97% purity), LiBH4 (90% purity) and sodium borohydride combination with XRD pattern matching to derive the best e $ (NaBH4, 98% purity) powders from Sigma Aldrich [40] were crystal structure solution of LiBH AB. used as received. The xLiBH4/AB mixtures in varied molar ratios (x ¼ 0.5e2) were mechanically milled under an argon (99.999% purity) atmosphere using a Fritsch 7 Planetary mill at 3. Results and discussion 200 rpm for 3 h. In the control experiment, the 1:1 NaBH4/AB was milled under identical conditions. All sample handlings 3.1. Synthesis and structure of LiBH4$NH3BH3 were carried out in an argon (99.999% purity)-filled glovebox equipped with a circulative purification system, in which the A systematic study of the series of xLiBH4/AB samples ＜ ¼ e H2O/O2 levels were typically 0.1 ppm. (x 0.5 2) found that different lithium borohydride ammonia The decomposition behaviour of the post-milled xLiBH4/AB borane complexes might be formed upon changing the molar samples was examined by synchronous thermogravimetry, ratio of the component phases. As shown in Fig. 1, mechanical ¼ $ differential scanning calorimetry, mass spectroscopy (TG, milling the mixtures of x 2 molar ratio produces (LiBH)2 AB, DSC, MS, Netzsch 449C Jupiter/QMS 403C) and volumetric which agrees well with the early finding [34]. But for the 1:1 method. In the thermal analyses, the samples with a typical LiBH4/AB sample, milling-induced solid-phase reaction amount of w2 mg were heated to 450 C at a ramping rate of produces a new crystalline phase. Here, the nearly complete À 2 Cmin 1 under a flowing argon (99.999% purity) atmosphere. consumption of the starting materials suggests its formula of The temperature-programmed desorption (TPD) and LiBH$AB. When 1 < x < 2 applied, the post-milled sample for $ $ isothermal measurements were carried out in a carefully 3 h is a mixture of (LiBH)2 AB and LiBH AB phases. To gain calibrated Sievert’s type apparatus under an initial pressure insight into the formation of the new LiBH$AB phase, we ＜100 Pa. In a typical TPD run, the sample with an amount of further examined the phase evolution in the milling process of w À1 100 mg was heated to 450 C at a ramping rate of 2 C min the 1:1 LiBH4/AB sample. As shown in Fig. 2, the XRD pattern of and held at this temperature until hydrogen desorption the post-milled LiBH4/AB sample for 0.5 h showed the $ $ completed.