With relatively high gravimetric and volumetric hydrogen capacities and low hydrogen operating pressures, borohydrides are being investigated for their potential use as solid-state hydrogen storage media. This work focuses on investigating the hydrogen sorption mechanisms for LiBH4-based low-melting-point borohydride mixtures (e.g. 0.62LiBH4-0.38NaBH4, 0.75LiBH4-0.25KBH4), and their destabilized systems using selected additives.
Solid solutions and bimetallic borohydride are found in the as-prepared 0.62LiBH4-0.38NaBH4 and 0.75LiBH4-0.25KBH mixtures, respectively. Under Ar, the 0.62LiBH4-0.38NaBH4 mixture releases 10.8 wt.% of hydrogen at 650 °C; whilst the 0.75LiBH4-0.25KBH4 mixture releases 8.9 wt.% of hydrogen at 700 °C. Their dehydrogenation peak temperatures are strongly affected by Na+ or K+ and therefore higher than LiBH4. These mixtures have poor cycling stabilities. Additives, such as micron-sized SiO2 and nano-sized Ni, cannot affect their melting points; but they cause lower dehydrogenation temperatures, decrease the hydrogen evolution, and facilitate the formation of metal dodecaborates. Besides, the addition of nano-sized Ni cannot significantly improve the cycling stability; however, it leads to partial reversible LiBH4.
Therefore, a further compositional optimization with respect to the rehydrogenation conditions, in parallel with the use of nano-confinement of the mixture via an infiltration approach, is needed before practical use of a low-melting-point alkali metal borohydride mixture
