Promotional Effect of Aluminum on MgH<SUB>2</SUB>+LiBH<SUB>4</SUB> Hydrogen Storage Materials

Abstract

The effect of Al addition on the reversibility of a MgH2+2LiBH4 hydrogen storage mixture was examined in order to improve the mixture’s requirement for a hydrogen atmosphere even in dehydrogenation. The experiments using high pressure differential scanning calorimetry and X-ray powder diffraction confirmed that a MgH2+Al+4LiBH4 mixture can reversibly dehydrogenate and rehydrogenate below 773 K under mild conditions of 0.1 MPa H2 for dehydrogenation and 4.0 MPa H2 for rehydrogenation. Moreover, thermogravimetry tests revealed that this mixture starts hydrogen desorption at about 530 K, which is 80 K lower than the corresponding temperature for the MgH2+2LiBH4 mixture, and desorbs 9.5 mass% of hydrogen below 773 K. Thus, the addition of Al improves not only the reversibility of the reaction but also dehydrogenation kinetics. The hydrogen desorption of the mixture occurs by three steps, which includes the formation of Mg-Al alloys by the reaction of MgH2 and metallic Al followed by the formation of Mg1−xAlxB2 by the reaction of the Mg-Al alloys and LiBH4. Al in this mixture suppresses the formation of metallic Mg and accelerates the formation of Mg1−xAlxB2 from B produced by dehydrogenation of LiBH4. Mg1−xAlxB2 is derived from partial substitution of Al for Mg in MgB2, which contributes to reversible hydrogenation and dehydrogenation of MgH2+2LiBH4.