Mg/MgH2 hydrogen storage system destabilized by recyclable AlH3–NaBH4 composite

While borohydrides, such as NaBH4, were often used as supplements to improve hydrogen storage properties of Mg/MgH2 systems, they have long suffered from high decomposition temperature and irreversible dehydrogenation process. Here, we report that NaBH4 can reversibly serve as a hydrogen storage host and reactant for Mg/MgH2 systems under mild reaction conditions with the help of Al/AlH3. 90 wt%MgH2–5 wt.%AlH3–5 wt.%NaBH4 (M-5AB) has been successfully synthesized using the conventional mechanical alloying technique. The dehydrogenation activation energy and enthalpy are 20% and 9% reduced than those of pure Mg/MgH2. After 10 hydrogen absorption and desorption cycles, the hydrogen storage capacity of M-5AB can reach 6.35 wt%. The X-ray diffraction (XRD) and the transmission electron microscope (TEM) measurements revealed that the interface of additives and Mg/MgH2 decompose to Mg17Al12, MgAlB4 and NaH phases. The Mg17Al12 and MgAlB4 phases reduces the barrier of free energies of hydrogenated and dehydrogenated states, helping NaBH4 to recover after rehydrogenation. These discoveries indicate that Al species can boost the decomposition and reformation of NaBH4, providing a wider degree of freedom for the material design of Mg-based hydrogen storage materials.