Magnesium borohydride, Mg(BH4)2 has among the highest hydrogen capacity and lowest enthalpy of dehydrogenation of the Group I and II borohydrides. Recently, it has been widely explored as a potential hydrogen storage material for onboard applications of PEM fuel cells. The cycling of 11.2 wt% hydrogen through the reversible hydrogenation of MgB2 to Mg(BH4)2 stands as the highest hydrogen storage capacity that has been experimentally demonstrated for any material that can be directly hydrogenated. While the cycling to MgB2 requires extreme reaction conditions (i.e, dehydrogenation &gt;400 oC and re-hydrogenation at 400 oC and 900 atm H2), we have found that Mg(BH4)2 undergoes selective, reversible dehydrogenated to Mg(B3H8)2 at 250 oC. We have also found that further improvement in the hydrogen cycling performance of the system can be achieved with etherates of the magnesium boranes and/or partial substitution of magnesium by alternative metals. For example, heating a 1:2 molar mixture of Mg(B3H8)2(THF)2 and MgH2 to 200 oC under 50 atm H2 for only 2h leads to nearly quantitative hydrogenation to the borohydride. We now wish to report that in the presence of sub-stoichiometric amounts of a variety of additives, much higher cycling capacities can be achieved through the reversible dehydrogenation of Mg(BH4)2 to Mg(B10H10) at moderate conditions (ie dehydrogenation at 180 oC, re-hydrogenation at 200 oC and 110 atm H2). The details of our continuing studies of this system by a combination of XRD, DSC, and PCT as well as 11B and 1H NMR analysis will be presented together with a discussion of our understanding of the role of the additives in enhancing the kinetics of the reversible dehydrogenation and an analysis of the progress that has been made towards the development of a practically viable magnesium borohydride based material that will undergo rapid, reversible loss of ~8 wt% hydrogen.

High capacity hydrogen cycling between magnesium borohydride and magnesium boranes under moderate conditions

255th National Meeting (2018)

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