Sorption properties of the NaBH4/MgH2 system: dehydrogenation mechanism and pathway

Nanostructured NaBH4/MgH2 composites have established themselves as promising materials for hydrogen storage applications due to their high gravimetric capacity, large hydrogen volumetric density and rather low dehydrogenation temperature compared to that one corresponding to the single compounds. Actually, further research on the NaBH4/MgH2 system could lead to an enhanced understanding of more complex reactive hydride composites, such as Ca(BH4)2/MgH2 or LiBH4/MgH2.

As-received NaBH4 and MgH2 powders were mixed, in a 2 to 1 molar ratio (2NaBH4/MgH2) and ball-milled to obtain nanostructured composites. The milling processes were carried out for diverse times, under Ar atmosphere, in a Spex mill with a ball-to-powder mass ratio of 10:1.

In-situ synchrotron x-ray powder diffraction indicates that the dehydrogenation process starts at around 210 C, with the desorption of the MgH2 to Mg, and rpoceeds with the chemical dismutation of NaBH4 in NaH and a possible intermediate specie, such as Na2B12H12. In fact, solid-NMR seems to confirm the existence of the transitional compound. However, the temperature onset of the dehydrogenation process of the NaBH4 counterpart has not yet been fully elucidated and, since it is difficutls to establish from either x-ray diffraction or thermogravimetric analyiss, magnetic hcaracterization is proposed as an alternative technique, which takes advantage of the superconducting nature of the MgB2 phase, to further study dehydrogenation processes.