Abstract
Boranes of low molecular weight are crystalline materials that have been much investigated over the past decade in the field of chemical hydrogen storage. In the present work, six of them have been selected to be studied by in situ synchrotron X-ray thermodiffraction. The selected boranes are ammonia borane NH3BH3 (AB), hydrazine borane N2H4BH3 (HB), hydrazine bisborane N2H4(BH3)2 (HBB), lithium LiN2H3BH3 (LiHB) and sodium NaN2H3BH3 (NaHB) hydrazinidoboranes, and sodium triborane NaB3H8 (STB). They are first investigated separately over a wide range of temperature (80–300 K), and subsequently compared. Differences in crystal structures, the existence of phase transition, evolutions of unit cell parameters and volumes, and variation of coefficients of thermal expansion can be observed. With respect to AB, HB and HBB, the differences are mainly explained in terms of molecule size, conformation and motion (degree of freedom) of the chemical groups (NH3, N2H4, BH3). With respect to LiHB, NaHB and STB, the differences are explained by a stabilization effect favored by the alkali cations via M···H interactions with four to five borane anions. The main results are presented and discussed herein.
Highlights
Chemical hydrides and especially boron- and/or nitrogen-based materials have shown to be potential solid-state hydrogen storage materials owing to high gravimetric hydrogen storage capacities [1]
At 240 K, the unit cell volume decreases by 0.85%
The replacement of the NH3 moiety of ammonia borane NH3 BH3 (AB) by N2 H4 leads to H4 BH3 (HB), and the addition of a BH3 group to HB leads to hydrazine bisborane N2H4(BH3)2 (HBB)
Summary
Chemical hydrides and especially boron- and/or nitrogen-based materials have shown to be potential solid-state hydrogen storage materials owing to high gravimetric hydrogen storage capacities [1]. These theoretical hydrogen densities are among the highest. Another example is ammonium borohydride [NH4 ][BH4 ]. Borane complexes L-BH3 have been known about for 70–80 years. It is generally considered that HBB is a derivative of HB, which is itself a derivative of AB. In that sense, they could be seen as being quite similar. HB and HBB have received much less attention until recently; we have initiated a major study aiming at re-visiting the fundamentals of these pristine boranes [9,10]. One of the phases of this study was in situ synchrotron X-ray thermodiffraction
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