Magnesium borohydride as a hydrogen storage material: Properties and dehydrogenation pathway of unsolvated Mg(BH4)2

The decomposition of crystalline magnesium borohydride upon heating was studied using thermal desorption, calorimetry, in situ X-ray diffraction, and solid state NMR. Hydrogen release from Mg(BH4)2 occurs in at least four steps via formation of several polyborane intermediate species and includes an exothermic reaction yielding crystalline MgH2 as an intermediate. The decomposition products may be only partially recharged after the very first step and also via hydrogenation of Mg metal. The intermediate formation of amorphous MgB12H12, was confirmed by 11B NMR. A four-stage pathway for the thermal decomposition of Mg(BH4)2 is proposed.