SYNERGISTIC EFFECT OF LiBH4 + MgH2 AS A POTENTIAL REVERSIBLE HIGH CAPACITY HYDROGEN STORAGE MATERIAL

The destabilisation of high capacity complex hydrides through the formation of an intermediary compound of lower enthalpy, allows system thermodynamics to be tailored. The destabilisation of lithium tetrahydridoborate with magnesium hydride is reported here, of a non-stoichiometric 0.3:1 molar ratio which gives a capacity of 9.8 wt.% with magnesium hydride, providing a higher capacity than can be achieved using metal or oxide additions. In-situ neutron diffraction (ND) was performed on the LiBD(4) : MgD(2) 0.3:1 system under dynamic vacuum and sealed vessel conditions, this allowed alternative reaction pathways to be elucidated for these different conditions. ND on heating to 730 degrees C under dynamic vacuum showed initial catalysis of LiBD(4) by Mg, this caused partial decomposition to form LiD, B, and D(2). Magnesium was then shown to destabilize the LiD through formation of dual alloys; Mg(0.816)Li(0.184) and Mg(0.70)Li(0.30). ND performed on a sample under sealed conditions did not show decomposition products of LiBD(4) until cooling from liquid phase after heating to 730 degrees C. This suggests a limiting pressure of deuterium (estimated at 30 bar) evolved during MgD(2) decomposition prevented the sample decomposing. Only after heating to temperatures above the melting point of Mg was the destabilization of LiBD(4) observed. We therefore postulate that this system progresses through one of two mechanisms according to pressure regime, and therefore under a controlled pressure the system may offer alternative destabilization mechanisms to those previously reported.