SYNERGISTIC EFFECT OF LiBH_4+ MgH_2AS A POTENTIAL REVERSIBLE HIGHCAPACITY HYDROGEN STORAGE MATERIAL

摘要

The destabilisation of high capacity complex hydrides through the formation of anintermediary compound of lower enthalpy, allows system thermodynamics to be tailored. Thedestabilisation 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 neutrondiffraction (ND) was performed on the LiBD_4: MgD_20.3:1 system under dynamic vacuum andsealed vessel conditions, this allowed alternative reaction pathways to be elucidated for thesedifferent conditions. ND on heating to 730 under dynamic vacuum showed initial catalysis ofLiBD4 by Mg, this caused partial decomposition to form LiD, B, and D_2.Magnesium was thenshown to destabilize the LiD through formation of dual alloys; Mg_(0.8 16)Li_(0. 184) and Mg_(0. 70)Li_(0.30).NDperformed on a sample under sealed conditions did not show decomposition products of LiBD_4until cooling from liquid phase after heating to 730. This suggests a limiting pressure ofdeuterium (estimated at 30 bar) evolved during MgD_2decomposition prevented the sampledecomposing. Only after heating to temperatures above the melting point of Mg was thedestabilization of LiBD_4observed. We therefore postulate that this system progresses through oneof two mechanisms according to pressure regime, and therefore under a controlled pressure thesystem may offer alternative destabilization mechanisms to those previously reported.