Lithium Hydrazinidoborane: A Polymorphic Material
with Potential for Chemical Hydrogen Storage
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Abstract
Herein, we describe the synthesis
and characterization (chemical,
structural, and thermal) of a new crystal phase of lithium hydrazinidoborane
(LiN<sub>2</sub>H<sub>4</sub>BH<sub>3</sub>, LiHB), which is
a new material for solid-state chemical hydrogen storage. We put in
evidence that lithium hydrazinidoborane is a polymorphic material,
with a stable low-temperature phase and a metastable high-temperature
phase. The former is called β-LiHB and the latter α-LiHB.
Results from DSC and XRD showed that the transition phase occurs at
around 90 °C. On this basis, the crystal structure of the novel
β-LiHB phase was solved. The potential of this material for
solid-state chemical hydrogen storage was verified by TGA, DSC, and
isothermal dehydrogenations. Upon the formation of the α-LiHB
phase, the borane dehydrogenates. At 150 °C, it is able to generate
10 wt % of pure H<sub>2</sub> while a solid residue consisting of
polymers with linear and cyclic units forms. Reaction mechanisms and
formation of bis(lithium hydrazide) of diborane [(LiN<sub>2</sub>H<sub>3</sub>)<sub>2</sub>BH<sub>2</sub>]<sup>+</sup>[BH<sub>4</sub>]<sup>−</sup> as a reaction intermediate are tentatively
proposed to highlight the decomposition of β-LiHB in our conditions