New insights into the thermal desorption of the 2LiNH2 + KBH4 + LiH mixture

• In situ 2-D synchrotron X-ray powder diffraction investigation on 2LiNH2 + KBH4 + LiH.
• Qualitative and quantitative phase analysis by the Rietveld method.
• Occurrence of the metastable monoclinic Li3BN2 polymorph at 380 °C.
• Coexistence and relative stability of monoclinic and tetragonal Li3BN2 polymorphs.
• Comparison between the thermodynamics predictions and experimental studies.

In situ two-dimensional synchrotron X-ray powder diffraction investigation combined with Rietveld method data analysis were performed in order to yield a complete and quantitative phases structure evolution of the polycrystalline mixture 2LiNH2 + KBH4 + LiH during H2 desorption. While a first-principles, purely thermodynamics approach of the system predicted a single dehydrogenation step reaction at relatively low temperatures, it is assessed experimentally that the reaction occurs in two steps with first the formation of Li2NH at ca. 230 °C due to the reaction between LiNH2 and LiH plus hydrogen and ammonia evolution, followed by an additional reaction of the resulting phases with KBH4 at 360 °C, which releases hydrogen and leads to the formation of the monoclinic and tetragonal Li3BN2 polymorphs. Besides pointing out possible limits of a purely thermodynamics approach inevitably relying exact knowledge of experimental quantities, it is concluded that before assuming it viable for on-board vehicle use, additional stoichiometries may be worth of investigation in order to assess any existence of lower hydrogen desorption temperature of such system.

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