Thermal decomposition behaviors of magnesium borohydride doped with metal fluoride additives

The thermal decomposition behaviors of Magnesium borohydride [Mg(BH4)2] and metal fluoride doped mixtures were studied by temperature programmed desorption measurement/mass spectrometry (TPD/MS), differential scanning calorimetry (DSC) and in situ transmission electron microscope (TEM) observations. The decomposition and release of hydrogen proceeded through several distinct steps, including two polymorphic transitions, ionic Mg(BH4)2 melting with solid Mg-B-H amorphous phase formation and Mg-B-H decomposition. The addition of additives such as CaF2, ZnF2 and TiF3 resulted in a decrease in the hydrogen release temperature. ZnF2 and TiF3 reduced the initial hydrogen release temperature to ca. 50 °C. However, hydrogen release during the transformation from γ-Mg(BH4)2 to the amorphous Mg-B-H compounds at ca. 300 °C was only 4.5 wt.% in contrast to 9.8 wt.% for the direct decomposition of pure Mg(BH4)2. TEM observations confirmed that ZnF2 and TiF3 reacted with amorphous Mg-B-H compounds rather than acting as catalysts.