Hydrogen storage properties in (LiNH₂)₂-LiBH ₄-(MgH₂)_x mixtures (X = 0.0-1.0)

We have recently reported the synthesis and properties of a novel hydrogen storage composition comprised of a 2:1:1 molar ratio of three hydride compounds: lithium amide (LiNH₂), lithium borohydride (L1BH₄), and magnesium hydride (MgH₂)- This new ternary mixture possesses improved hydrogen (de)sorption attributes (relative to the individual compounds and their binary mixtures), including facile low-temperature kinetics, ammonia attenuation, and partial reversibility. Comprehensive characterization studies of its reaction pathway revealed that these favorable hydrogen storage properties are accomplished through a complex multistep hydrogen release process. Here, we expound on our previous findings and determine the impact of MgH₂ content on the resulting hydrogen storage properties by examining a series of (LiNH₂)₂-LiBH₄-(MgH ₂)_x reactant mixtures (i.e., 2:1: X molar ratio) where X = 0, 0.15, 0.25, 0.40, 0.50, 0.75, and 1.0. Specifically, we characterize each starting composition (after ball-milling) using powder X-ray diffraction (PXRD) and infrared spectroscopy (IR) and find that addition of MgH₂ facilitates a spontaneous milling-induced reaction, introducing new species (Mg(NH₂)₂ and LiH) into the hydride composition. We additionally measure the relative hydrogen and ammonia release amounts for each mixture using temperature-programmed desorption mass spectrometry (TPD-MS) and find that ammonia liberation is suppressed for increasing values of X (<0.1 wt % NH₃ for X = 1). Kinetic hydrogen desorption data reveal a low-temperature reaction step (centered at ∼ 160 °C) for all MgH ₂-containing samples which grows in intensity for larger values of X (up to ∼4.0 wt % H₂ for X = 1). Finally, we characterize desorbed samples to investigate the dependence of X (MgH₂ amount) on the resulting distribution of observed product phases. These data are used to understand how MgH₂ contributes to and impacts the low- and high-temperature hydrogen release events through comparing theoretical (based on the previously proposed reaction set) and observed desorption data for these reactions.