Solid Aluminum Borohydrides as Perspective Hydrogen Stores

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