NbF5 additive improves hydrogen release from magnesium borohydride

The dehydrogenation properties of Mg(BH4)2 with various additives (SiO2, VCl3, CoCl2 and NbF5) were investigated. The addition of NbF5 significantly improved the extent of hydrogen release as well as the kinetics. While neat Mg(BH4)2 starts to release hydrogen >270 °C, Mg(BH4)2 with NbF5 begins hydrogen release ∼75 °C, as confirmed by mass spectrometry and thermogravimetry. The maximum hydrogen yield of Mg(BH4)2, obtained in the presence of 15 wt% NbF5, was 3.7, 7.4, 10.0, 11.4 wt% for 150, 250, 300 and 350 °C, respectively. Using pXRD, we confirmed that the final crystalline product at 300 °C from Mg(BH4)2 + 15 wt% NbF5 was Mg, while it was MgH2 for neat Mg(BH4)2. Solid state 11B NMR analysis of Mg(BH4)2 with 15 wt% NbF5 at 300 °C showed significant selectivity toward the formation of Mg(B12H12) as intermediate, while neat Mg(BH4)2 showed β-Mg(BH4)2, Mg(B2H6) as well as some Mg(B12H12). Our results demonstrate that NbF5 is a promising additive to provide high hydrogen yield values from Mg(BH4)2 at moderate temperatures <300 °C.

► NbF5 improves kinetics of hydrogen release from Mg(BH4)2. ► Hydrogen release from Mg(BH4)2 with NbF5 additive begins at ∼75 °C. ► NbF5 causes Mg(BH4)2 phase transformation to generate favorable intermediate, MgB12H12.