Study on the hydrogen storage properties and reaction mechanism of NaAlH4–Mg(BH4)2 (2:1) with and without TiF3 additive

Abstract

In this work, hydrogen storage properties and reaction mechanism of NaAlH4–Mg(BH4)2 (2:1) composites system with and without additive have been investigated. The secondary-hydride system was found to initiate a transformation of the NaAlH4/Mg(BH4)2 to Mg(AlH4)2/NaBH4 during ball milling, and there was a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the systems, which corresponded to the decomposition of Mg(AlH4)2, MgH2, and NaBH4. Temperature-programmed desorption results showed that the TiF3-doped NaAlH4–Mg(BH4)2 (2:1) composite sample started to release hydrogen at about 75 °C, which was 60 °C lower than that of undoped NaAlH4–Mg(BH4)2 (2:1) composite sample. In addition, the reaction pathway of the NaAlH4–Mg(BH4)2 (2:1) composite system, and the mechanisms that worked in this composite during the de/rehydrogenation process were determined by X-ray diffraction. The Kissinger analysis has shown that the apparent activation energy, EA, for decomposition of NaBH4 in the NaAlH4–Mg(BH4)2–TiF3 composite reduced to 139.85 kJ/mol compared with 155.73 kJ/mol in NaAlH4–Mg(BH4)2 composite. It is believed that the enhancement of the de/rehydrogenation properties of undoped NaAlH4/Mg(BH4)2 was attributed to the formation of intermediate compounds, including Mg–Al and Mg–Al–B alloys, upon dehydrogenation, which changed the thermodynamics of the reactions by altering the de/rehydrogenation pathway. Meanwhile, as for the doped sample, the TiF3 component played a catalytic role through the formation of Ti-containing and F-containing catalytic species, which might have promoted the interaction of Mg(AlH4)2 and NaBH4, and thus, further improved the dehydrogenation of the NaAlH4–Mg(BH4)2 (2:1) system.