Solid Aluminum Borohydrides for Prospective Hydrogen Storage

Abstract

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

Keywords: aluminum; crystal structures; high-energy materials; hydrides; hydrogen storage.