Structural evolution upon decomposition of the LiAlH 4 + LiBH 4 system S. Soru a , A. Taras a , C. Pistidda b , C. Milanese c , C. Bonatto Minella d , E. Masolo a , P. Nolis g , M.D. Baró e , A. Marini c , M. Tolkiehn f , M. Dornheim b , S. Enzo a , G. Mulas a , S. Garroni a,⇑ a Department of Chemistry and Pharmacy, University di Sassari and INSTM, Via Vienna 2, I-07100 Sassari, Italy b Institute of Materials Research, Materials Technology, Helmholtz-Zentrum Geesthacht, Max-Planck, Str. 1, D-21502 Geesthacht, Germany c Pavia H2 Lab, C.S.G.I. & Dipartimento di Chimica, Sezione di Chimica Fisica, Università di Pavia, Viale Taramelli 16, I-27100 Pavia, Italy d IFW Dresden, Institute for Metallic Materials, Helmholtzstrasse 20, D-01069 Dresden, Germany e Universitat Autònoma de Barcelona, Departament de Física, E-08193 Bellaterra, Spain f DESY Synchrotron, Beam Line D3, Hamburg, Germany g Servei de Ressonància Magnètica Nuclear and Departament de Química, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain article info Article history: Available online xxxx Keywords: Hydrogen storage materials LiBH 4 Synchrotron Radiation Powder X-ray Diffraction Solid State Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) abstract In the present work we focus the attention on the phase structural transformations occurring upon the desorption process of the LiBH 4 + LiAlH 4 system. This study is conducted by means of manometric– calorimetric, in situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD) and ex situ Solid State Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. The desorption reaction is characterized by two main dehydrogenation steps starting at 320 and 380 °C, respectively. The first step corresponds to the decomposition of LiAlH 4 into Al and H 2 via the formation of Li 3 AlH 6 whereas the second one refers to the dehydrogenation of LiBH 4 (molten state). In the range 328–380 °C, the mol- ten LiBH 4 reacts with metallic Al releasing hydrogen and forming an unidentified phase which appears to be an important intermediate for the desorption mechanism of LiBH 4 –Al-based systems. Interestingly, NMR studies indicate that the unknown intermediate is stable up to 400 °C and it is mainly composed of Li, B, Al and H. In addition, the NMR measurements of the annealed powders (400 °C) confirm that the desorption reaction of the LiBH 4 + Al system proceeds via an amorphous boron compound. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In the field of the hydrogen-based fuel cells technology, enormous efforts have been devoted to the development of mate- rials capable to reversibly store high amounts of hydrogen with favorable thermodynamic and kinetic properties [1–3]. Currently, large interest is addressed towards a class of materials defined as tetraborohydrides, due to their high gravimetric hydrogen storage densities [4]. Among them, LiBH 4 is considered one of the most attractive and promising materials for automotive application, be- cause of its high hydrogen gravimetric capacity of 18.5 wt.% [2,5,6]. However, due to its high thermodynamic stability (T des > 500 °C) and sluggish sorption kinetics, LiBH 4 does not meet the require- ments set for on-board hydrogen storage [7]. Recently, in order to tailor the thermodynamics and the kinetics of LiBH 4 de-hydro- genation process, different approaches were proposed which can be classified in three categories: the addition of catalysts, the nanoconfinement into scaffolds and the destabilization of the tetrahydroborate by combination with a hydride phase. Doping with several additives including halides, oxides and pure metals effectively reduces the dehydriding temperature of LiBH 4 [8–12]. For example, Au et al. verified that the halides TiF 3 , TiCl 3 and ZnCl 2 , when added to LiBH 4 , form unstable transition me- tal borohydride species which contribute to drastically reduce the thermal desorption temperature of the doped LiBH 4 from 300 °C to less than 100 °C [9]. Destabilization of LiBH 4 was also achieved by the addition of different oxides with the following order of efficiency: Fe 2 O 3 >V 2 O 5 > Nb 2 O 5 > TiO 2 > SiO 2 [10]. More recently, Pendolino and coauthors demonstrated that the desorption temperature of LiBH 4 can be decreased from 500 °C to 350 °C by the addition of boron [12]. Another method is represented by the confinement of LiBH 4 in mesoporous scaffolds, nanotubes and carbon aerogels [13–17]. LiBH 4 was successfully infiltrated into the mesoporous channels of SBA-15 under hydrogen pressure [13]. For the as-prepared LiBH 4 /SBA-15 nanocomposite, the initial desorption temperature was 150 °C [14]. Vajo et al. reported that fast desorption kinetics of LiBH 4 was achieved (50 times at 300 °C) when it was confined within a nanoporous carbon scaffold. However, despite recent progress, the employment of scaffolds drastically decreases the hydrogen storage density of the whole system. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.027 ⇑ Corresponding author. Tel.: +39 079 229524; fax: +39 079 229559. E-mail address: sgarroni@uniss.it (S. Garroni). Journal of Alloys and Compounds xxx (2013) xxx–xxx Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom Please cite this article in press as: S. Soru et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2013.12.027