Unexpected kinetic effect of MgB2 in reactive hydride composites containing complex borohydrides

Complex borohydrides of light metals are promising hydrogen storage materials due to their high hydrogen capacity. However, they exhibit two main drawbacks: their high thermodynamic stability and their slow kinetics. In the present work, the effect of various reactants on the formation kinetics of complex borohydrides is investigated. It is found that the kinetic barriers for the formation of LiBH4, NaBH4 and Ca(BH4)2 are drastically reduced when MgB2 is used instead of B as starting material. Since this kinetic enhancement is observed in all borohydride studied so far, the observed effect is attributed to the higher reactivity of B in MgB2 to form [BH4]− complexes. In addition, by using MgB2 instead of elemental B, the corresponding reaction enthalpies are reduced by about 10 kJ/mol H, while the high gravimetric hydrogen capacities are largely preserved, i.e. LiBH4 + MgH2 with 11.4 wt%, Ca(BH4)2 + MgH2 with 8.3 wt%, and NaBH4 + MgH2 with 7.8 wt%.