Solid Aluminum Borohydrides for Prospective Hydrogen Storage

Metal borohydrides are intensively researched as high
capacity hydrogen storage materials. Aluminum is a cheap, light and
abundant element and Al3+ can be a template for reversible
dehydrogenation. However, Al(BH4)3, containing 16.9 weight % of
hydrogen, has a low boiling point, is explosive on air and has poor
storage stability. We present a new family of mixed-cation
borohydrides M[Al(BH4)4], all solid at ambient conditions. Their
thermal decomposition properties show diverse behavior: Al(BH4)3 is
released for M = Li+, Na+, while heavier derivatives evolve hydrogen
and diborane. NH4[Al(BH4)4], containing protic and hydridic
hydrogens, has the lowest decomposition temperature of 35 °C and
yields Al(BH4)3·NHBH and hydrogen. The decomposition
temperatures, correlated with cations' ionic potential, show that
M[Al(BH4)4] are in the most practical stability window. This family of
solids with convenient and versatile properties puts aluminum
borohydride chemistry in the mainstream of the hydrogen storage
research, e.g. for the development of reactive hydride composites
with increased hydrogen content.