In-situ Raman study of the thermal decomposition of LiBH 4
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1216-W06-05
In-situ Raman study of the thermal decomposition of LiBH4 Daniel Reed and David Book School of Metallurgy and Materials, University of Birmingham, Birmingham, B15 2TT, UK ABSTRACT There is interest in the potential of lithium borohydride for use as a hydrogen storage material, either on its own or in the form of a reactive composite with a metal hydride. In this study insitu Raman spectroscopy has been used to identify structural changes in LiBH4 and the reaction products and intermediates formed during thermal decomposition. After heating LiBH4 through its phase change (118°C) shifts in Raman peak position and peak width were observed, which showed good agreement with previous studies 1,2. Upon further heating under 1 bar flowing Ar to 500°C, the in situ formation, from liquid lithium borohydride, of lithium dodecaborane (Li2B12H12) and amorphous boron at 350°C and 385°C respectively was observed. INTRODUCTION With relatively high gravimetric and volumetric hydrogen storage capacities, borohydrides have attracted interest as potential hydrogen storage media. Lithium borohydride has a maximum theoretical gravimetric hydrogen storage density of 18.5 wt%, and has been shown to be reversible when heated to 600°C in 350 bar hydrogen 3. LiBH4 combined with MgH2 has been shown to significantly lower the desorption temperature and introduce reversibility, but also reduce the hydrogen storage capacity 4,5.Upon heating, LiBH4 goes through a transformation at 108°C from an orthorhombic (o-LiBH4) to a hexagonal (h-LiBH4) phase 6. Then melting occurs at 270°C followed by the start of the thermal decomposition of LiBH4 at 200°C with the second major decomposition observed at 450°C with the release of a total of about 14 wt% 7. It is hoped that a greater understanding of the possible reaction paths during decomposition, may lead to the development of LiBH4-based materials that can absorb and desorb hydrogen under less extreme conditions (on their own, or in the form of reactive hydride composites). Thermal decomposition was first thought to follow the reaction shown in equation 1 7. LiBH4 → LiH + B + 3/2H2
(1)
First principle studies by Ohba et al. considered many possible intermediates, including LiB3H8 and Li2BnHn (n=5-12), and showed that Li2B12H12 was theoretically the most stable reaction product; they also calculated a vibrational spectrum for Li2B12H12 8. Subsequently, Orimo et al. 9, performed room temperature Raman measurements on LiBH4 that had been heated to above 430°C, and found spectra that corresponded to amorphous boron and Li2B12H12 (equations 2 and 3). Equation 2 corresponds to a weight loss of 10 wt% and 56 kJ/molH2 (which is about 20 kJ/molH2 less than predicted [5]), and equation 3 with 4 mass% and 125 kJ/molH2. The presence of An/2B12H12 was also confirmed in MAS-NMR studies by Hwang et al. for A= Li, Mg and LiSc 10 .
LiBH4 ↔ 1/12Li2B12H12 + 5/6LiH + 13/12H2 ↔ LiH + B + 3/2H2
(2) (3)
Previous studies 1,11 have identified 23 of the 36 Raman active bands present in the lowtemperature o-LiBH4
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