Structural evolution upon decomposition of the LiAlH4+LiBH4 system

Abstract

a b s t r a c tIn 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 manometriccalorimetric, 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 molten 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.

Confined LiBH4–LiAlH4 in nanopores of activated carbon nanofibers

“…Onset dehydrogenation temperature of LiAlH 4 reduces bỹ 50 C after milling with LiBH 4 , while that of LiBH 4 is comparable to bulk material, in agreement with the previous works of LiBH 4 eLiAlH 4 [18,19]. It was reported that by compositing LiBH 4 with Al or LiAlH 4 , although reduction of onset dehydrogenation temperature of LiBH 4 was not achieved, reversibility of LiBH 4 was improved due to the formation of AlB 2 instead of amorphous boron (a-B) [17,18].…”

“…For milled LiBH 4 eLiAlH 4 , separately two-step decomposition of LiAlH 4 and LiBH 4 in the ranges of 100e200 and 300e500 C releases totally 8.7 wt.% H 2 ( Fig. 3(C)), approaching to the previous work of Soru et al [19]. With respect to theoretical hydrogen storage capacity of LiBH 4 eLiAlH 4 composite of 10.12 wt.% based on equation (7) [19], 86% of theoretical value is obtained from milled LiBH 4 eLiAlH 4 .…”

“…7, 11 B MAS NMR spectrum of bulk LiBH 4 and milled LiBH 4 eLiAlH 4 show a typical resonance of BH 4 À group centered at À41.7 ppm, approaching to the previous studies [19,32,46]. For as-prepared powder sample of nano LiBH 4 eLiAlH 4 eACNF, the resonance of LiBH 4 splits into two peaks, i.e., sharp and broad peaks at À38.2 and À42.7 ppm, respectively (Fig.…”

“…Due to the formation of the latter phases, reversibility of LiBH 4 could be achieved under relatively mild physical condition (T ¼ 400 C and P(H 2 ) ¼ 100 bar) as compared with bulk LiBH 4 . Furthermore, Soru et al [19] reported that during dehydrogenation (T ¼ 400 C under 1 bar Ar) of LiBH 4 eLiAlH 4 composite, molten LiBH 4 reacted with Al particles, obtained from the decomposition of LiAlH 4 and Li 3 AlH 6 , to produce LieAleBeH phase instead of AlB 2 as in the previous systems.…”

“…Composite of LiBH 4 eLiAlH 4 un-doped and doped with titanium (Ti)-based catalysts (TiF 3 , TiO 2 , and TiCl 3 ) were reported [9,16e19]. It was found that milled LiBH 4 eLiAlH 4 liberated hydrogen into two steps, where LiAlH 4 and LiBH 4 decomposed separately, in the temperature ranges of about 118e210 and 300e600 C [16,17,19]. By doping 5 mol% of TiF 3 and TiO 2 into 2LiBH 4 eLiAlH 4 composite, onset dehydrogenation temperature of LiAlH 4 decreased by 64 and 27 C, respectively, whereas that of LiBH 4 remained approximately as high as bulk LiBH 4 .…”