Study on the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 ternary-hydride system

Abstract

In this paper, we report the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 (1:1:1) ternary-hydride system prepared by ball milling. It was found that during ball milling, the NaAlH4/MgH2/LiBH4 combination converted readily to the mixture of LiAlH4/MgH2/NaBH4 and there is a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the system, which corresponds to the decomposition of LiAlH4, MgH2, and NaBH4, respectively. The onset dehydrogenation temperature of MgH2 in this system is observed at around 275 °C, which is over 55 °C lower from that of as-milled MgH2. Meanwhile, NaBH4-relevant decomposition showed significant improvement, starts to release hydrogen at 370 °C, which is reduced by about 110 °C compared to the as-milled NaBH4. The second and third steps decomposition enthalpy of the system were determined by differential scanning calorimetry measurements and the enthalpies were changed to be 61 and 100 kJ mol−1 H2 respectively, which are smaller than that of MgH2 and NaBH4 alone. From the Kissinger plot, the apparent activation energy, EA, for the decomposition of MgH2 and NaBH4 in the composite was reduced to 96.85 and 111.74 kJ mol−1 respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including Li–Mg, Mg–Al, and Mg–Al–B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway.