Insights into magnesium borohydride dehydrogenation mechanism from its partial reversibility under moderate conditions

Magnesium borohydride, Mg(BH4)2, is a potential hydrogen storage material because of its high hydrogen capacity and favorable thermodynamic stability comparing to other borohydrides. Although various intermediates, which influence the hydrogen storage properties, have been explored during the hydrogen desorption process of Mg(BH4)2, the in-depth dehydrogenation mechanism is still unclear. Herein, partial reversibility of Mg(BH4)2 and its dehydrogenation mechanism are investigated by isothermal cycles. It is found that Mg(BH4)2 can release 7.06 wt% H2 at 270°C for 3-day duration. After rehydrogenation, 3.61 wt% H2 is released during the second dehydrogenation. In the third and fourth cycles of hydrogen desorption, 3.54 and 3.24 wt% H2 are released, respectively. By carefully analyzing de/rehydrogenated samples with nuclear magnetic resonance and Fourier transform infrared spectrometer methods, the MgB5H9, MgB2H6, MgB8H8, MgB9H9, and MgB12H12 borohydride intermediates can be confirmed. Among the intermediates, MgB5H9 and MgB9H9 possess reversibility to Mg(BH4)2, whereas MgB8H8 and MgB12H12 are irreversible under the current hydrogen absorption conditions of 120 bar hydrogen backpressure. Considering the content change trend of these intermediates during hydrogen de/absorption cycles, the partial dehydrogenation mechanism of Mg(BH4)2 is proposed. In general, there are two different kinds of borohydride intermediates consisting in the hydrogen de/absorption processes: the metastable kind, MgBxHy (x < y), and the stable kind, MgBzHz. During the early phase of dehydrogenation, Mg(BH4)2 tends to first decompose into MgBxHy, which possesses relatively high hydrogen content. Later, MgBzHz with relatively low hydrogen content will be transformed from both Mg(BH4)2 and MgBxHy and become the majority of the dehydrogenation products. Finally, MgBzHz will polymerize into intermediates with high ‘z’ values during further hydrogen desorption process, eventually MgB12H12. Such partial dehydrogenation mechanism of Mg(BH4)2 is assumed to be promoted to the general dehydrogenation process of Mg(BH4)2. Furthermore, the discovery of the reversible intermediates including MgB5H9 and MgB9H9 sheds light on the method of thermodynamically controlling the dehydrogenation conditions to selectively form reversible intermediates, for the purpose of enhancing the reversible properties of Mg(BH4)2.